fix: convert dragged file paths to @mentions for attachment (#382)

* fix: convert dragged file paths to @mentions for attachment

When non-image files are dragged into the terminal, the file path was
inserted as plain text and never attached. Now detected absolute paths
are converted to @mentions so they get picked up by the attachment system.

* test: add tests for drag-and-drop file path detection

* fix: multi-image drag-and-drop only showing last image

insertTextAtCursor read input and cursorOffset from the React closure,
which is stale when called in a synchronous loop (e.g. onImagePaste for
multiple dragged images). Now uses refs so each insertion chains on the
previous one.

* fix: quote Windows absolute paths to avoid MCP mention collision

Paths containing ':' (e.g. Windows drive letters) are now emitted in
quoted @"..." form so they don't match the MCP resource mention regex.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: decouple dragDropPaths from imagePaste and harden image checks

- Check image extension against the cleaned path (post quote/escape
  stripping) so quoted or backslash-escaped image drops are reliably
  routed to the image paste handler.
- Inline the image extension regex and drop the imagePaste/fsOperations
  imports so the module (and its tests) no longer pull in `bun:bundle`
  and the heavier fs wrapper chain. Use plain `fs.existsSync` for the
  on-disk check.
- Add tests covering quoted image paths, uppercase extensions,
  backslash-escaped image paths, escaped real files with spaces, mixed
  segments containing an image, quoted-nonexistent paths, and leading
  or trailing whitespace.

* test: verify dragged paths with an `@` segment are preserved

Adds a fixture under a scoped-package-style subdir (`@types/index.d.ts`)
so we exercise the realistic `node_modules/@types/...` drag case and
lock in that `extractDraggedFilePaths` returns the raw path unchanged —
the `@` inside the path must not collide with the mention prefix the
caller prepends downstream.

* test: parametrize dragDropPaths cases with test.each

Groups the 21 scenarios into four table-driven describes
(empty-result, single-path, multi-path, backslash-escaped) so that
adding a new case is a one-line row instead of a new `test()` block.
Fixture directories are now created synchronously at describe-load
time so their paths are available to the test.each tables, which are
built before any hook runs.

* test: add contract tests for @-mention extractor boundary

Pins the contract between `extractAtMentionedFiles` and
`extractMcpResourceMentions` so the MCP regex can't silently swallow
quoted file-path mentions.

These tests fail on current HEAD — 3 of 11 cases expose the regression
pointed out in the review on #382: `extractMcpResourceMentions`'s
trailing `\b` backtracks past the closing `"` of a quoted mention and
produces a ghost match for `@"C:\Users\..."`, `@C:\Users\...`, and
`@"/tmp/weird:name.txt"`. The remaining 8 cases lock in the behaviour
that must not change (legitimate `server:resource` mentions and plain
file-path mentions).

Committed failing on purpose as the first half of a test-then-fix
pair; the regex fix follows in a subsequent commit.

* fix: prevent MCP extractor from ghost-matching quoted/Windows paths

The MCP resource regex used `\b` as a trailing anchor with `[^\s]+`
character classes. On any quoted file mention containing a colon
(`@"C:\Users\me\file.txt"`, `@"/tmp/weird:name.txt"`), the engine
backtracked past the closing `"` to satisfy `\b`, producing a ghost
match that collided with `extractAtMentionedFiles`. Unquoted Windows
drive-letter paths (`@C:\Users\me\file.txt`) also matched because a
drive letter is structurally identical to an MCP `server:resource`
token.

Two guards:

1. `(?!")` right after `@` drops quoted tokens entirely, and adding
   `"` to the character classes blocks any mid-match backtracking.
2. A post-match filter discards `^[A-Za-z]:[\\/]` — a single-letter
   server followed by a path separator is always a Windows drive
   prefix, never a real MCP resource.

Legitimate MCP forms (`@server:resource/path`, plugin-scoped like
`@asana-plugin:project-status/123`, inline prose mentions) remain
matched and are pinned by the contract tests added in 04998d5.

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/PromptInput/PromptInput.tsx

@@ -67,6 +67,7 @@ import { isBilledAsExtraUsage } from '../../utils/extraUsage.js';
 import { getFastModeUnavailableReason, isFastModeAvailable, isFastModeCooldown, isFastModeEnabled, isFastModeSupportedByModel } from '../../utils/fastMode.js';
 import { isFullscreenEnvEnabled } from '../../utils/fullscreen.js';
 import type { PromptInputHelpers } from '../../utils/handlePromptSubmit.js';
+import { extractDraggedFilePaths } from '../../utils/dragDropPaths.js';
 import { getImageFromClipboard, PASTE_THRESHOLD } from '../../utils/imagePaste.js';
 import type { ImageDimensions } from '../../utils/imageResizer.js';
 import { cacheImagePath, storeImage } from '../../utils/imageStore.js';
@@ -1204,6 +1205,22 @@ function PromptInput({
     // Clean up pasted text - strip ANSI escape codes and normalize line endings and tabs
     let text = stripAnsi(rawText).replace(/\r/g, '\n').replaceAll('\t', '    ');
 
+    // Detect file paths from drag-and-drop and convert to @mentions.
+    // When files are dragged into the terminal, the terminal sends their
+    // absolute paths via bracketed paste. Image files are handled by the
+    // image paste handler upstream; here we handle non-image files by
+    // converting them to @mentions so they get attached on submit.
+    const draggedPaths = extractDraggedFilePaths(text);
+    if (draggedPaths.length > 0) {
+      const mentions = draggedPaths
+        .map(p => (p.includes(' ') || p.includes(':') ? `@"${p}"` : `@${p}`))
+        .join(' ');
+      // Ensure spacing around the mention(s) relative to existing input
+      const charBefore = input[cursorOffset - 1];
+      const prefix = charBefore && !/\s/.test(charBefore) ? ' ' : '';
+      text = prefix + mentions + ' ';
+    }
+
     // Match typed/auto-suggest: `!cmd` pasted into empty input enters bash mode.
     if (input.length === 0) {
       const pastedMode = getModeFromInput(text);
@@ -1245,12 +1262,23 @@ function PromptInput({
     if (isNonSpacePrintable(input, key)) return ' ' + input;
     return input;
   }, []);
+  // Ref mirrors cursorOffset for use in synchronous loops (e.g. multi-image
+  // paste) where React batches state updates and the closure value is stale.
+  const cursorOffsetRef = useRef(cursorOffset);
+  cursorOffsetRef.current = cursorOffset;
+
   function insertTextAtCursor(text: string) {
-    // Push current state to buffer before inserting
-    pushToBuffer(input, cursorOffset, pastedContents);
-    const newInput = input.slice(0, cursorOffset) + text + input.slice(cursorOffset);
+    // Use refs for input/cursor so back-to-back calls in the same event
+    // (e.g. onImagePaste loop for multiple dragged images) chain correctly
+    // instead of each reading the same stale closure values.
+    const currentInput = lastInternalInputRef.current;
+    const currentOffset = cursorOffsetRef.current;
+    pushToBuffer(currentInput, currentOffset, pastedContents);
+    const newInput = currentInput.slice(0, currentOffset) + text + currentInput.slice(currentOffset);
     trackAndSetInput(newInput);
-    setCursorOffset(cursorOffset + text.length);
+    const newOffset = currentOffset + text.length;
+    cursorOffsetRef.current = newOffset;
+    setCursorOffset(newOffset);
   }
   const doublePressEscFromEmpty = useDoublePress(() => {}, () => onShowMessageSelector());