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.

crates/g3-cli/src/lib.rs

@@ -18,6 +18,7 @@ mod task_execution;
 mod ui_writer_impl;
 mod utils;
 mod g3_status;
+mod completion;
 
 use anyhow::Result;
 use std::path::PathBuf;