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/normalizeUserDisplayParts.ts

@@ -1,4 +1,5 @@
 import type { Part } from '@opencode-ai/sdk/v2';
+import { readContextPart } from '@/lib/messages/contextParts';
 
 const GITHUB_ISSUE_CONTEXT_PREFIX = 'GitHub issue context (JSON)';
 const GITHUB_PR_CONTEXT_PREFIX = 'GitHub pull request context (JSON)';
@@ -96,6 +97,7 @@ export const normalizeUserDisplayParts = (parts: Part[], options?: { planModeEna
             const synthetic = (part as { synthetic?: boolean }).synthetic === true;
             if (!synthetic) return true;
             if (part.type !== 'text') return false;
+            if (readContextPart(part)) return true;
             const text = (part as { text?: unknown }).text;
             if (typeof text !== 'string') {
                 return false;
@@ -116,6 +118,27 @@ export const normalizeUserDisplayParts = (parts: Part[], options?: { planModeEna
                 const synthetic = rawPart.synthetic === true;
 
                 if (synthetic) {
+                    const contextPayload = readContextPart(part);
+                    if (contextPayload?.kind === 'github-issue' || contextPayload?.kind === 'github-pr') {
+                        // SAFETY: same display-only file-part shape the legacy
+                        // buildGitHubAttachmentPart produces; consumed by
+                        // FileAttachment, which matches on the mime type.
+                        return {
+                            type: 'file',
+                            mime: contextPayload.kind === 'github-issue'
+                                ? 'application/vnd.github.issue-link'
+                                : 'application/vnd.github.pull-request-link',
+                            filename: contextPayload.kind === 'github-issue'
+                                ? `Issue #${contextPayload.number}: ${contextPayload.title}`
+                                : `PR #${contextPayload.number}: ${contextPayload.title}`,
+                            url: contextPayload.url,
+                        } as Part;
+                    }
+                    if (contextPayload) {
+                        // Other context kinds render through UserContextPart.
+                        return part;
+                    }
+                    // Legacy messages: sniff the pre-metadata text format.
                     const attachmentPart = buildGitHubAttachmentPart(text);
                     if (attachmentPart) {
                         return attachmentPart;