feat(web,coder): arena pane — compare 2-6 AI competitors on same prompt

Arena is a new pane kind for competitive AI evaluation. A Battle runs
the same prompt against 2-6 Contestants across two concurrent lanes:
local lane (llama-swap models, serial) and cloud lane (parallel).

Added to all three registries: @boocode/contracts WsFrameSchema,
server InferenceFrame, and web WsFrame.

Backend (apps/coder):
- arena-runner: battle scheduler, lane classifier, benchmark, results
  writer, resume, user winner override
- arena-analyzer: two-stage digest→judge analysis on DEFAULT_MODEL
- arena-decisions: status transitions and resume logic (unit-tested)
- arena-analyzer-helpers: pure helper functions (unit-tested)
- arena-model-call: model call utility for analysis
- arena routes: create/get/list/stop/analyze/cross-examine/winner/diff
- schema: battles, contestants, cross_examinations tables (idempotent)
- remove old /api/arena* routes and tasks.arena_id column

Frontend (apps/web):
- ArenaLauncherDialog: battle type, prompt, contestant selection
- ArenaPane: live roster, streaming output, analysis, cross-exam
- DiffView: unified diff with line-by-line color for coding contests
- Winner override per-row dropdown (Trophy icon)
- battle_updated WS handler for live winner/analysis updates
- arena pane kind in Workspace, ChatTabBar, useSidebar

Cross-app:
- ArenaState and ArenaContestantShape/WsFrame types (contracts)
- battle_* frames in WsFrameSchema, InferenceFrame, and web WsFrame
- manifest.json written per battle results folder
- /Arena added to .gitignore

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/coder/src/services/dispatcher.ts

@@ -310,6 +310,9 @@ export function createDispatcher(deps: Deps): {
     const taskId = task.id;
     log.info({ taskId }, 'dispatcher: starting task (path A — native)');
 
+    // Declared before try so the catch block can write it back on the task row.
+    let chatId: string | null = null;
+
     try {
       // Mark running
       await sql`
@@ -318,26 +321,29 @@ export function createDispatcher(deps: Deps): {
         WHERE id = ${taskId}
       `;
 
-      // Create session + chat for this task
+      // Session setup: reuse a pre-created session (e.g. Q&A arena contestants
+      // whose persona is stamped on the session via agent_id) or create a fresh one.
       const model = task.model ?? config.DEFAULT_MODEL;
-      const sessionName = 'Task: ' + task.input.slice(0, 40);
-
-      const [session] = await sql<{ id: string }[]>`
-        INSERT INTO sessions (project_id, name, model, status)
-        VALUES (${task.project_id}, ${sessionName}, ${model}, 'open')
-        RETURNING id
-      `;
-      const sessionId = session!.id;
+      let sessionId: string;
+      if (task.session_id) {
+        sessionId = task.session_id;
+      } else {
+        const sessionName = 'Task: ' + task.input.slice(0, 40);
+        const [session] = await sql<{ id: string }[]>`
+          INSERT INTO sessions (project_id, name, model, status)
+          VALUES (${task.project_id}, ${sessionName}, ${model}, 'open')
+          RETURNING id
+        `;
+        sessionId = session!.id;
+        await sql`UPDATE tasks SET session_id = ${sessionId} WHERE id = ${taskId}`;
+      }
 
       const [chat] = await sql<{ id: string }[]>`
         INSERT INTO chats (session_id, name, status)
         VALUES (${sessionId}, 'Task execution', 'open')
         RETURNING id
       `;
-      const chatId = chat!.id;
-
-      // Link task to session
-      await sql`UPDATE tasks SET session_id = ${sessionId} WHERE id = ${taskId}`;
+      chatId = chat!.id;
 
       // Create user message + streaming assistant
       await sql<{ id: string }[]>`
@@ -382,7 +388,7 @@ export function createDispatcher(deps: Deps): {
         const summary = (msg?.content ?? '').slice(0, 500);
         await sql`
           UPDATE tasks
-          SET state = 'completed', ended_at = clock_timestamp(), output_summary = ${summary}, cost_tokens = ${costTokens}
+          SET state = 'completed', ended_at = clock_timestamp(), output_summary = ${summary}, cost_tokens = ${costTokens}, chat_id = ${chatId}
           WHERE id = ${taskId}
         `;
         log.info({ taskId, costTokens }, 'dispatcher: task completed (native)');
@@ -409,7 +415,7 @@ export function createDispatcher(deps: Deps): {
         const summary = (msg?.content ?? 'Inference failed').slice(0, 500);
         await sql`
           UPDATE tasks
-          SET state = 'failed', ended_at = clock_timestamp(), output_summary = ${summary}, cost_tokens = ${costTokens}
+          SET state = 'failed', ended_at = clock_timestamp(), output_summary = ${summary}, cost_tokens = ${costTokens}, chat_id = ${chatId}
           WHERE id = ${taskId}
         `;
         log.warn({ taskId, finalStatus }, 'dispatcher: task failed (native)');
@@ -419,7 +425,7 @@ export function createDispatcher(deps: Deps): {
       log.error({ taskId, err: errMsg }, 'dispatcher: task error (native)');
       await sql`
         UPDATE tasks
-        SET state = 'failed', ended_at = clock_timestamp(), output_summary = ${errMsg.slice(0, 500)}
+        SET state = 'failed', ended_at = clock_timestamp(), output_summary = ${errMsg.slice(0, 500)}, chat_id = ${chatId}
         WHERE id = ${taskId}
       `.catch(() => {});
     }