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/lib/opencode/client.ts

@@ -1,3 +1,4 @@
+import type { ContextPartMetadata } from '@/lib/messages/contextParts';
 import { createOpencodeClient, OpencodeClient } from "@opencode-ai/sdk/v2";
 import type { PermissionV2Request, PermissionV2Effect, PermissionV2Source } from "@opencode-ai/sdk/v2/client";
 import type { FilesAPI } from "../api/types";
@@ -792,6 +793,7 @@ class OpencodeService {
     additionalParts?: Array<{
       text: string;
       synthetic?: boolean;
+      metadata?: ContextPartMetadata;
       files?: Array<FileInputLite>;
     }>;
     messageId?: string;
@@ -842,11 +844,10 @@ class OpencodeService {
     if (params.additionalParts && params.additionalParts.length > 0) {
       for (const additional of params.additionalParts) {
         if (additional.text && additional.text.trim()) {
-          parts.push({
-            type: 'text',
-            text: additional.text,
-            ...(additional.synthetic ? { synthetic: true } : {}),
-          });
+          const additionalTextPart: TextPartInput = { type: 'text', text: additional.text };
+          if (additional.synthetic) additionalTextPart.synthetic = true;
+          if (additional.metadata) additionalTextPart.metadata = additional.metadata;
+          parts.push(additionalTextPart);
         }
         if (additional.files && additional.files.length > 0) {
           for (const file of additional.files) {
@@ -1193,7 +1194,7 @@ class OpencodeService {
     options?: {
       id?: string;
       save?: string[];
-      metadata?: Record<string, unknown>;
+      metadata?: ContextPartMetadata;
       source?: PermissionV2Source;
       agent?: string;
     }