Add tab completion for commands and file paths

Implement tab completion in interactive mode using rustyline:

- Command completion: /<TAB> shows all commands, /com<TAB> -> /compact
- File path completion: /run <TAB> completes file/directory paths
- Supports tilde expansion for home directory

Architecture is extensible for future semantic completions:
- /resume <TAB> -> session IDs (Phase 2)
- /rehydrate <TAB> -> fragment IDs (Phase 2)

New module: completion.rs with G3Helper struct implementing
rustyline's Completer trait.

Cargo.lock

@@ -3085,12 +3085,24 @@ dependencies = [
  "memchr",
  "nix",
  "radix_trie",
+ "rustyline-derive",
  "unicode-segmentation",
  "unicode-width 0.2.0",
  "utf8parse",
  "windows-sys 0.60.2",
 ]
 
+[[package]]
+name = "rustyline-derive"
+version = "0.11.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d66de233f908aebf9cc30ac75ef9103185b4b715c6f2fb7a626aa5e5ede53ab"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "ryu"
 version = "1.0.20"