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/message/parts/DOCUMENTATION.md

@@ -121,6 +121,14 @@ Why: only navigation tools use the compact static path; all other tools need obs
 ## Quick map of files in this folder
 
 - Text: `AssistantTextPart.tsx`, `UserTextPart.tsx`
+- User-attached context (inline code comments, terminal selections, browser
+  annotations, PR comments/checks): `UserContextPart.tsx`. `UserTextPart`
+  routes to it when the part's metadata carries an `openchamberContext`
+  payload (see `lib/messages/contextParts.ts`, which owns both the send-time
+  builder and the read-back parser). Linked GitHub issues/PRs are instead
+  converted to link file-parts in `normalizeUserDisplayParts.ts`. Legacy
+  pre-metadata messages still render via text sniffing (`<terminal_context>`
+  blocks, `GitHub issue context (JSON)` prefixes).
 - Tools: `ToolPart.tsx`, `ToolPartDiffPreview.tsx`, `PlainDiffFallback.tsx`, `ProgressiveGroup.tsx`, `toolPresentation.tsx`, `toolRenderUtils.ts`, `ToolRevealOnMount.tsx`
 - Reasoning/justification: `ReasoningPart.tsx`, `JustificationBlock.tsx`
 - Status/placeholders: `WorkingPlaceholder.tsx`, `SessionActiveSpinner.tsx`, `MigratingPart.tsx`, `BusyDots.tsx`