Add workspace file mentions to the composer

[sabari245]

Add workspace file mentions to the composer

This PR adds @ file and directory mentions to the chat composer.

Typing @ or @query opens a workspace-relative picker. Selecting a result inserts a Markdown link such as:

[composer.rs](comet-file:crates/ui/src/composer.rs)
[components](comet-file:crates/ui/src/components/)

The editor renders these links as atomic file or folder chips while keeping the raw Markdown prompt as the single source of truth.

Highlights

• Searches files and directories within the active repository or a validated linked worktree.
• Supports case-insensitive, whitespace-separated subsequence matching.
• Prioritizes filename matches, recent files, changed files, and shallow paths.
• Respects Git and repository ignore rules while allowing hidden files.
• Returns up to eight results after an 80 ms debounce.
• Uses shortest unique path suffixes for duplicate filenames.
• Supports keyboard navigation, dismissal, tooltips, wrapped chips, deletion, undo, and selection as a single atomic unit.

Safety and lifecycle

• The client sends only a chat or space identity; the engine resolves and validates the checkout.
• Searches outside known repository roots and linked worktrees are rejected.
• Filesystem walks are cancellable, serialized per checkout, and bounded by a six-second deadline.
• Blocking filesystem work runs on a disposable OS thread rather than Tokio’s shared blocking pool.
• Request generations prevent stale results from replacing newer UI state.
• Mention behavior is enabled only for the chat composer and is cleared when the token, draft, chat, submission state, or wizard mode changes.

GPUI structure

• Composer handles token detection, search requests, selection, dismissal, and popover rendering.
• ComposerInput handles Markdown projection, atomic chip editing, layout, painting, hit testing, and tooltips.
• The menu reuses existing popover and menu helpers and is anchored to the visible token position.

Validation

• cargo fmt --all -- --check
• git diff --check
• cargo test --workspace --all-targets

All passed. Six environment-dependent tests remain ignored, and only pre-existing warnings were emitted.

Focused tests cover root validation, ignore rules, hidden files, directories, ranking, cancellation, concurrent callers, stale responses, Markdown validation, duplicate labels, atomic editing, wrapped geometry, tooltips, and menu navigation.

Reviewed locally with no blocking findings. Safe to merge.

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[sabari245]

Screenshots

File mention search and result picker:

Inserted directory mention with its full-path tooltip:

[wingleeio]

Reviewed the full diff with a security lens (external-contributor PR). Engine surface is well-defended: SearchFiles resolves roots from the engine's own synced rows, validates them by canonicalized equality against the repo root or registered worktrees (stricter than the existing ListFolders), caps query length and results, bounds walks with deadlines on disposable threads, and only relative paths ever cross the RPC boundary. The ignore/globset/bstr additions are the standard ripgrep-family crates. Composer chip parsing rejects absolute paths, dot-segments, backslashes, control chars, and non-canonical encodings, with tests for the hostile variants. No process/network/env/unsafe usage, no hidden Unicode.

Follow-ups pushed to the branch: the merge with main (one conflict in composer.rs, resolved in favor of the themed selection/caret tokens plus the mention wash) and transcript-side chip rendering for sent messages (projection cached per entry change, gpui line-layout cache keeps streaming frames cheap; ordinary prompts stay on the zero-allocation path).

Verified at head: cargo fmt --check clean, 723/723 workspace tests pass. Known follow-up: transcript chips have no hover tooltip yet — InteractiveText::tooltip makes that a small addition.