feat(chat): structured context attachments with metadata round-trip

Every user-attached context item (diff/file/plan comments, terminal
selections, browser annotations, PR comments and failed checks, linked
issues/PRs, and new chat-quote comments from the selection menu) is now
sent as its own synthetic text part carrying an openchamberContext
metadata payload. The model-facing text keeps the previous wording; the
timeline reads the metadata back and renders each item as a context card
instead of raw prompt text. Legacy messages still render via the old
text sniffing.

The selection menu gains a Comment option with an inline multiline
input, the quoted fragment stays highlighted while commenting, and on
mobile the input overlays the composer pill by rendering inside the
composer form. Add to chat is renamed Add to input; the menu is
restyled and the mobile Copy tile removed. Terminal drafts move their
terminal id out of the language field (persisted-draft migration v3),
and the dead preview-console source is deleted.

packages/ui/src/components/chat/composer/DOCUMENTATION.md

@@ -124,10 +124,14 @@ and the send path reading the same grammar.
   drawn caret through a class it only writes while applying an update, so the
   selection has to be the update that follows the focus.
 - `submit/buildOutgoingMessage.ts` flattens queued messages, the composer text,
-  inline comments and context into OpenCode's one-primary-plus-parts shape. The
-  oldest queued message becomes primary; **inline comments attach to the last
-  body the user authored** rather than becoming their own part; PR instructions
-  precede the PR diff.
+  context drafts and linked references into OpenCode's one-primary-plus-parts
+  shape. The oldest queued message becomes primary. **Every attached context
+  item (inline comments, terminal selections, browser annotations, PR context,
+  linked issue/PR) becomes its own synthetic text part carrying structured
+  metadata** built by `lib/messages/contextParts.ts`; the timeline reads that
+  metadata back to render context blocks. PR instructions precede the PR diff.
+  Queueing a message leaves context drafts in their store on purpose — the send
+  that later delivers the queue consumes them.
 - `state/useComposerDraft.ts` — a draft belongs to a (runtime, directory,
   session) identity. Writes are debounced while typing but forced at every edge
   where the page may stop running, because a pending timer is not a saved