feat(queue): queue a message with everything the composer had attached

Queueing captured only the text and files. Context chips (inline comments,
terminal selections, browser annotations, PR comments and checks, quotes,
linked issue/PR/Linear references, pending synthetic parts) stayed in the
composer and only left with the next manual send, so a queued message the
server delivered went out without them and the chips rode an unrelated
message later.

A queued message now carries what the composer would have sent: the text
with its agent mention stripped and file mentions resolved into
attachments, the attached context as structured parts, and the skill
instruction derived from the text. The server delivers those parts in the
composer's order, the VS Code auto-send does the same, and editing a queued
message puts the chips and linked references back. A failed queue restores
the composer completely. Snapshots and broadcasts omit the captured
context like attachment payloads; a take returns it.

Claude-Session: https://claude.ai/code/session_01HB9wdLQoZX2vfyDjwv6Rso

packages/ui/src/lib/messages/contextParts.ts

@@ -315,6 +315,12 @@ const contextPayloadSchema = z.discriminatedUnion('kind', [
     }),
 ]);
 
+/**
+ * Part metadata carrying a context payload, for parsing at a trust boundary
+ * (a queued message coming back from the server, for instance).
+ */
+export const contextPartMetadataSchema = z.object({ [CONTEXT_METADATA_KEY]: contextPayloadSchema });
+
 /** The subset of a message part that context read-back inspects. */
 export type ContextCarrierPart = { type: string } & Pick<TextPart, 'metadata'>;