Context-action @-mentions are lost on a cold start (the staged note survives, the mention doesn't)

[DaveTseng2019]

What

On a cold start (no claude session running), a context action's staged .txt note survives — the attachment tray re-sends it when the CLI connects — but the at_mentioned it pushes alongside is dropped and never re-sent. The note arrives in the composer talking about code that was never mentioned.

Observed while verifying something else on #26, in the experimental hive on 18.9.12105.275 with main@1.18.1 (CLI 2.1.229). Feed from two Explains fired with nothing running:

[info] Explain: no Claude session - launching one. Staged items deliver when it connects.
[info] attach: staged 'explain-Program.cs.txt' (~29 tok) - will @-mention it when Claude connects.
[warn] Explain: Claude isn't connected.                     <- the whole-file mention, gone
[info] Launched Claude Code in VS's native Terminal window (...)
...
[info] client connected from 127.0.0.1:11208 (authorized)
[info] attach: @-mentioned '.claude/attachments/explain-Program.cs.txt' in the Claude composer.

So the composer ends up holding "Explain the file RefMaze/Program.cs (mentioned alongside this note)…" with no @Program.cs next to it.

Every action that mentions a range is affected the same way: Explain (whole file), Fix Errors (file, or the selection's line range), Generate Documentation / Add Comments (the function's span), Fix This Test (the test method's span).

Add to Chat / Alt+K is the sharpest case: it's the one action that doesn't call EnsureSessionLaunching, so with no session it neither launches nor stages anything — the click is a no-op plus a warning line. (That part is from reading ContextActions.AddToChatAsync, not something I ran.)

Why

1.18.0 made actions launch a session when none is running precisely so a click isn't wasted — but the launch takes a few seconds, and the mention is pushed immediately after it, into a socket that has no client yet. The staged note has a flush path (AttachmentService.FlushUnsentAsync on ConnectionChanged); the mention has none, so the half that carries the ground truth is the half that's lost.

Implementation notes

• The tray already solves this shape, and Add to Chat in Solution Explorer: @-mention selected files and folders #35 leans on it: stage → chip → send, with unsent items flushed on connect and re-sendable by clicking the chip. Routing the actions' mentions through the same queue would close the gap and get click-to-re-mention for free.
• The one thing that doesn't fit today is the line range: AttachmentItem has no lineStart/lineEnd, and SendAsync always emits a whole-file at_mentioned. Either add optional line fields to the item (and pass them through in SendAsync), or keep a small pending-mention queue next to SelectionService.MentionAsync and flush it from the same ConnectionChanged event. The first keeps one queue and one chip vocabulary; the second leaves the tray untouched.
• Ordering matters: the note says "mentioned alongside this note", so the mention should flush after the staged .txt — the existing flush walks the tray in insertion order, which gives that for free if the mention lives in the same queue.
• Worth deciding at the same time whether Add to Chat should launch a session like its siblings do. Once mentions survive a cold start, EnsureSessionLaunching in AddToChatAsync becomes a one-line change with the same behaviour as the rest of the flyout.

Happy to take this one if you'd like — it's adjacent to #35, and I have the cold-start repro set up.