feat(cosh): add Tab-completion for shell mode

[kongche-jbw]

• Implement useShellCompletion hook with command completion ($PATH executables, 30s cache) and file-system path completion (supports ~/, absolute and relative paths; directories suffixed with /)
• Integrate shell completion into InputPrompt: Tab triggers/accepts completion, arrow keys navigate suggestions, Enter always falls through to execute the command without interception

Description

Add Tab-key auto-completion for cosh shell mode (!).

When the user enters shell mode by typing !, pressing Tab now:

• Completes command names (executables on $PATH) when the cursor
  is on the first token; results are cached for 30 s to avoid repeated
  directory scans.
• Completes file-system paths on subsequent tokens, supporting
  ~/-prefixed, absolute and relative paths; directories are displayed
  with a trailing / and kept open for further tab-navigation.
• Does not block Enter — the command is always executable
  regardless of completion state.
• Does not interfere with the existing @-file (AT-mode)
  completion inside shell mode; Tab falls through to the regular
  handler when AT suggestions are already visible.

Related Issue

closes #22

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional change)
• Performance improvement
• CI/CD or build changes

Scope

• cosh (copilot-shell)
• sec-core (agent-sec-core)
• skill (os-skills)
• sight (agentsight)
• Multiple / Project-wide

Checklist

• I have read the Contributing Guide
• My code follows the project's code style
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly
• For cosh: Lint passes, type check passes, and tests pass
• For sec-core (Rust): cargo clippy -- -D warnings and cargo fmt --check pass
• For sec-core (Python): Ruff format and pytest pass
• For skill: Skill directory structure is valid and shell scripts pass syntax check
• For sight: cargo clippy -- -D warnings and cargo fmt --check pass
• Lock files are up to date (package-lock.json / Cargo.lock)

Testing

1. Start cosh (npm run start in src/copilot-shell)
2. Type ! to enter shell mode
3. Type a partial command (e.g. gi) and press Tab → command list
   appears; ↑/↓ to navigate, Tab again to accept
4. Type a partial path (e.g. ls ~/) and press Tab → file-system
   completions appear; directories are suffixed with /
5. Press Tab on a directory suggestion to navigate deeper
6. Press Enter at any time → command executes immediately
7. In regular (non-shell) mode, type @filename → dropdown appears,
   Tab accepts as before (no regression)

Additional Notes

• PATH scan is done synchronously but only on the first Tab press;
  results are cached for 30 s at module level.
• Max suggestions per trigger: 100 (prevents UI overflow).
• New file: packages/cli/src/ui/hooks/useShellCompletion.ts

[samchu-zsl]

A performance concern: will processing a large number of files - say, 10k or more - in the directory impact UI responsiveness?

[kongche-jbw]

A performance concern: will processing a large number of files - say, 10k or more - in the directory impact UI responsiveness?

Thanks for raising this! Since completions are only triggered on explicit Tab press (not on every keystroke), which limits exposure. I've filed this as a follow-up optimization item — moving the directory scan to an async path (or a Worker thread) to keep the UI fully responsive regardless of directory size.

[samchu-zsl]

Basic functionality LGTM. Remaining performance optimizations will be tracked in the issue.

[samchu-zsl]

Progress on this optimization is being tracked in GitHub issue #138.