web(coder UI): ChatInput migration + Thinking render + DiffPanel route fix

Bundles in-progress working-tree UI work not authored this session (CoderPane ChatInput migration, AgentComposerBar/CoderMessageList/tab-bar/sidebar/pane refinements, provider icons) with this session's changes to the same files: MessageBubble renders a collapsible 'Thinking' block from reasoning_text/reasoning_parts (surfacing ACP agent_thought_chunk + native reasoning), and the DiffPanel approve/reject calls are repointed to the real /api/coder/pending/:id/apply and /reject routes (the old /sessions/:id/pending/:id/approve|reject paths did not exist).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/api/types.ts

@@ -182,10 +182,14 @@ export interface Message {
   // majority of messages.
   metadata: MessageMetadata | null;
   // v1.13.1-C: reasoning content captured from models that stream reasoning
-  // tokens separately (qwen3.6 etc.). Backend populates from message_parts;
-  // optional on the wire — frontend doesn't render this yet (reserved for
-  // a v1.14 UI surface).
+  // tokens separately (qwen3.6 etc.) and from external agents over ACP
+  // (agent_thought_chunk). Backend populates from message_parts; rendered by
+  // MessageBubble as a collapsible "Thinking" block.
   reasoning_parts?: Array<{ text: string }> | null;
+  // Coder wire shape pre-joins reasoning_parts into a single string
+  // (CoderPane/CoderMessageList) and streams it live via reasoning_delta
+  // frames. MessageBubble reads whichever of the two is present.
+  reasoning_text?: string | null;
   // v1.11: anchored rolling compaction fields. Optional on the wire so that
   // older API responses (or test fixtures) parse without explicit nulls.
   //   summary       — true on the assistant row that holds the active