feat(tui): add workspace file autocomplete

[svadrutk]

HumanLayer Task

Why the change

Typing @ in the TUI can now find and insert workspace-relative file paths without changing Kit's plain-text prompt or ACP submission model.

Special things to note

• The picker activates only for a typed @ at the start of the prompt or after whitespace; pasted text and email-like input stay ordinary text.
• Tab completes the selected path, while Enter keeps its existing submit behavior.
• fff-search is isolated behind a private adapter, and indexing or search failures close the picker without losing prompt text.

Change outline

The new adapter keeps third-party search state outside the editor and application state.

 Cargo.toml / Cargo.lock       # add fff-search and bump Kit to 0.1.119
 src/tui/
 ├── editor.rs                # expose cursor and replace UTF-8-safe ranges
+├── file_search.rs           # own FFF setup, queries, results, and errors
 ├── app.rs                   # own trigger, picker state, revisions, and keys
 ├── mod.rs                   # run blocking index/search work asynchronously
+└── ui.rs                    # render the picker next to the active composer

Search results follow the existing action/update boundary and remain plain prompt text.

typed eligible @
  App inserts @ and emits SearchFiles(query, revision)
    blocking worker lazily starts WorkspaceFileSearch
      fff-search indexes and searches the canonical workspace root
    FileMatches(revision, matches)
      stale revision -> discard
      current revision -> render near composer
  Up/Down -> choose result
  Tab -> replace @query with @relative/path
  Enter -> existing prompt submission

Screenshots