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/utils/attachments.ts

@@ -2793,11 +2793,30 @@ export function extractAtMentionedFiles(content: string): string[] {
 export function extractMcpResourceMentions(content: string): string[] {
   // Extract MCP resources mentioned with @ symbol in format @server:uri
   // Example: "@server1:resource/path" would extract "server1:resource/path"
-  const atMentionRegex = /(^|\s)@([^\s]+:[^\s]+)\b/g
+  //
+  // Two guards against Windows-path / quoted-file collisions (see
+  // `attachments.extractors.test.ts`):
+  //
+  // 1. `(?!")` right after `@` drops quoted tokens entirely. The earlier
+  //    form (without the lookahead and with `[^\s]` character classes)
+  //    backtracked past the closing `"` at the `\b` anchor and produced
+  //    ghost matches like `"C:\Users\...\file.txt` for any quoted file
+  //    mention containing a colon.
+  // 2. The `"` added to the character classes is belt-and-braces: even
+  //    if the lookahead were later removed or bypassed, the engine can
+  //    no longer consume a quote character mid-match.
+  const atMentionRegex = /(^|\s)@(?!")([^\s"]+:[^\s"]+)\b/g
   const matches = content.match(atMentionRegex) || []
 
-  // Remove the prefix (everything before @) from each match
-  return uniq(matches.map(match => match.slice(match.indexOf('@') + 1)))
+  return uniq(
+    matches
+      .map(match => match.slice(match.indexOf('@') + 1))
+      // Post-match filter: a single-letter "server" followed by `:\` or
+      // `:/` is always a Windows drive-letter prefix, never a real MCP
+      // resource. This covers the unquoted `@C:\Users\...` case that
+      // the regex alone cannot disambiguate from `@server:resource`.
+      .filter(m => !/^[A-Za-z]:[\\/]/.test(m)),
+  )
 }
 
 export function extractAgentMentions(content: string): string[] {