Extend tab completion to path-like prefixes anywhere in line

Path completion now works for:
- ./<TAB> - current directory
- ../<TAB> - parent directory
- ~/<TAB> - home directory
- /<TAB> (not at start of line) - root directory

Command completion (/<TAB>) only triggers at the start of the line.
If no command matches, falls through to path completion (e.g., /etc).

Quote-aware word extraction handles paths with spaces:
- "~/My Files/<TAB>" works correctly

Added tests for:
- Path prefix detection
- Word extraction with quotes
- Command vs path disambiguation

crates/g3-cli/src/completion.rs

@@ -1,8 +1,12 @@
 //! Tab completion support for g3 interactive mode.
 //!
 //! Provides:
-//! - Command completion for `/` commands
-//! - File path completion for `/run <path>`
+//! - Command completion for `/` commands (at start of line)
+//! - File path completion for paths anywhere in the line:
+//!   - `./` - current directory
+//!   - `../` - parent directory  
+//!   - `~/` - home directory
+//!   - `/` (not at start) - root directory
 //! - Extensible for future semantic completions (sessions, fragments, etc.)
 
 use rustyline::completion::{Completer, FilenameCompleter, Pair};
@@ -30,7 +34,7 @@ const COMMANDS: &[&str] = &[
 
 /// Helper struct for rustyline that provides tab completion.
 pub struct G3Helper {
-    /// File path completer for `/run` command
+    /// File path completer
     file_completer: FilenameCompleter,
 }
 
@@ -40,6 +44,52 @@ impl G3Helper {
             file_completer: FilenameCompleter::new(),
         }
     }
+
+    /// Find the start of the current "word" being typed, respecting quotes.
+    /// Returns (word_start, word) where word_start is the byte index.
+    fn extract_word<'a>(&self, line: &'a str, pos: usize) -> (usize, &'a str) {
+        let line_to_cursor = &line[..pos];
+        
+        // Look backwards for the start of the word
+        // A word starts after a space (unless quoted)
+        let mut word_start = 0;
+        let mut in_quotes = false;
+        let mut quote_char = ' ';
+        
+        for (i, c) in line_to_cursor.char_indices() {
+            if in_quotes {
+                if c == quote_char {
+                    in_quotes = false;
+                }
+            } else {
+                match c {
+                    '"' | '\'' => {
+                        in_quotes = true;
+                        quote_char = c;
+                        word_start = i;
+                    }
+                    ' ' | '\t' => {
+                        // Next char starts a new word
+                        word_start = i + 1;
+                    }
+                    _ => {}
+                }
+            }
+        }
+        
+        (word_start, &line_to_cursor[word_start..])
+    }
+
+    /// Check if a word looks like a path prefix
+    fn is_path_prefix(&self, word: &str) -> bool {
+        word.starts_with("./")
+            || word.starts_with("../")
+            || word.starts_with("~/")
+            || word.starts_with('/')
+            || word == "."
+            || word == ".."
+            || word == "~"
+    }
 }
 
 impl Default for G3Helper {
@@ -57,33 +107,52 @@ impl Completer for G3Helper {
         pos: usize,
         ctx: &Context<'_>,
     ) -> Result<(usize, Vec<Pair>), ReadlineError> {
-        // Only complete up to cursor position
         let line_to_cursor = &line[..pos];
         
-        // Case 1: `/run <path>` - complete file paths
-        if line_to_cursor.starts_with("/run ") {
-            // Delegate to file completer
-            return self.file_completer.complete(line, pos, ctx);
-        }
+        // Extract the current word being typed
+        let (word_start, word) = self.extract_word(line, pos);
         
-        // Case 2: `/rehydrate <fragment_id>` - future: complete fragment IDs
-        // Case 3: `/resume <session>` - future: complete session IDs
-
-        // Case 4: `/` commands - complete command names
-        if line_to_cursor.starts_with('/') {
-            let prefix = line_to_cursor;
+        // Case 1: Command completion - `/` at the very start of the line
+        // Only complete commands if we're typing the first word and it starts with `/`
+        if word_start == 0 && word.starts_with('/') && !word.contains(' ') {
+            // Check if this looks like a command (no path separators after the first /)
+            let after_slash = &word[1..];
+            if !after_slash.contains('/') {
+                // This is a command like "/com" not a path like "/etc"
                 let matches: Vec<Pair> = COMMANDS
                     .iter()
-                .filter(|cmd| cmd.starts_with(prefix))
+                    .filter(|cmd| cmd.starts_with(word))
                     .map(|cmd| Pair {
                         display: cmd.to_string(),
                         replacement: cmd.to_string(),
                     })
                     .collect();
+                
+                if !matches.is_empty() {
                     return Ok((0, matches));
                 }
+                // If no command matches, fall through to path completion
+                // (e.g., "/etc" should complete as a path)
+            }
+        }
         
-        // No completion for regular prompts
+        // Case 2: Path completion for path-like prefixes
+        // Delegate to FilenameCompleter which handles:
+        // - Tilde expansion
+        // - Quote handling for spaces
+        // - Proper escaping
+        if self.is_path_prefix(word) || word_start > 0 && line_to_cursor[word_start..].starts_with('/') {
+            return self.file_completer.complete(line, pos, ctx);
+        }
+        
+        // Case 3: Check if we're after a command that takes a path argument
+        if line_to_cursor.starts_with("/run ") 
+            || line_to_cursor.starts_with("/rehydrate ")
+        {
+            return self.file_completer.complete(line, pos, ctx);
+        }
+
+        // No completion for regular text
         Ok((pos, vec![]))
     }
 }
@@ -134,6 +203,52 @@ mod tests {
         assert!(matches.iter().any(|m| m.replacement == "/stats"));
     }
 
+    #[test]
+    fn test_path_prefix_detection() {
+        let helper = G3Helper::new();
+        
+        assert!(helper.is_path_prefix("./"));
+        assert!(helper.is_path_prefix("./src"));
+        assert!(helper.is_path_prefix("../"));
+        assert!(helper.is_path_prefix("~/"));
+        assert!(helper.is_path_prefix("~/Documents"));
+        assert!(helper.is_path_prefix("/etc"));
+        assert!(helper.is_path_prefix("."));
+        assert!(helper.is_path_prefix(".."));
+        assert!(helper.is_path_prefix("~"));
+        
+        assert!(!helper.is_path_prefix("hello"));
+        assert!(!helper.is_path_prefix("src"));
+    }
+
+    #[test]
+    fn test_extract_word_simple() {
+        let helper = G3Helper::new();
+        
+        let (start, word) = helper.extract_word("hello world", 11);
+        assert_eq!(start, 6);
+        assert_eq!(word, "world");
+    }
+
+    #[test]
+    fn test_extract_word_with_path() {
+        let helper = G3Helper::new();
+        
+        let (start, word) = helper.extract_word("edit ./src/main.rs", 18);
+        assert_eq!(start, 5);
+        assert_eq!(word, "./src/main.rs");
+    }
+
+    #[test]
+    fn test_extract_word_quoted() {
+        let helper = G3Helper::new();
+        
+        // Quoted path with spaces
+        let (start, word) = helper.extract_word("edit \"./My Files/doc", 20);
+        assert_eq!(start, 5);
+        assert_eq!(word, "\"./My Files/doc");
+    }
+
     #[test]
     fn test_no_completion_for_regular_input() {
         let helper = G3Helper::new();
@@ -145,4 +260,16 @@ mod tests {
         assert_eq!(start, 11);
         assert!(matches.is_empty());
     }
+
+    #[test]
+    fn test_slash_at_start_is_command() {
+        let helper = G3Helper::new();
+        let history = rustyline::history::DefaultHistory::new();
+        let ctx = Context::new(&history);
+
+        // "/h" at start should complete to commands
+        let (start, matches) = helper.complete("/h", 2, &ctx).unwrap();
+        assert_eq!(start, 0);
+        assert!(matches.iter().any(|m| m.replacement == "/help"));
+    }
 }