chat: remove Pi runner — route all chat/pipeline/channels through Claude

Stage 1 of removing Pi (Claude-only). Cuts the non-Claude branches in the chat
WS handler, pipeline executor, and channel send-and-await; deletes the Pi
sidecar, its ecosystem entry, pi-bridge, and the Pi model-listing spawn (now a
static Claude tier list). Adds a guard coercing any legacy non-claude-code model
preference to the Claude default so old settings don't break chat or jobs.
Removes the dead no-op session-save REST route and stale Pi docs.

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

src/servers/api/tasks/pipeline-executor.ts

@@ -8,7 +8,6 @@ import { getTaskByDirName } from './task-files';
 import { getHomeDirForRole, getHomeDir } from '../../data-path';
 import { resolveBaseCwd } from '../pi/websocket';
 import { SANDBOX_HOME } from '../../sidecar/sandbox';
-import * as sidecar from '../../sidecar-registry';
 import { sendClaudeCodeStreaming } from '../../channels/send-claude-code';
 import type { PiEvent, MessageCost } from '../pi/types';
 import * as jobManager from './pipeline-job-manager';
@@ -169,32 +168,18 @@ async function runAgenticStep({ userId, email, username, role, taskDirName, prom
     }, WAITING_INTERVAL_MS);
 
     try {
-      if (isClaudeCode) {
-        const handle = await sendClaudeCodeStreaming({
-          userId,
-          email,
-          username,
-          prompt,
-          sessionKey: sessionId,
-          cwd,
-          model,
-          role,
-          onEvent,
-        });
-        cleanup = handle.kill;
-      } else {
-        const unsub = sidecar.onPiEvent((evtSessionId, event) => {
-          if (evtSessionId === sessionId) onEvent(event);
-        });
-        cleanup = () => {
-          unsub();
-          sidecar.killPi(sessionId);
-        };
-
-        await sidecar.spawnPi({ sessionId, email, userId, username, role, cwd, model });
-        sidecar.sendPiPrompt(sessionId, prompt, randomUUID());
-      }
-
+      const handle = await sendClaudeCodeStreaming({
+        userId,
+        email,
+        username,
+        prompt,
+        sessionKey: sessionId,
+        cwd,
+        model,
+        role,
+        onEvent,
+      });
+      cleanup = handle.kill;
     } catch (err) {
       settle(() => { cleanup?.(); reject(err); });
     }
@@ -489,7 +474,9 @@ export async function executePipeline({ userId, email, username, role, taskDirNa
   }
 
   const baseCwd = resolveBaseCwd(email, role, cwd);
-  const model = modelOverride || (await resolveModel(userId));
+  let model = modelOverride || (await resolveModel(userId));
+  // Claude-only: coerce any legacy non-Claude task-model preference to the Claude default.
+  if (!model.startsWith('claude-code')) model = DEFAULT_MODEL;
   const totalCost: MessageCost = { inputTokens: 0, outputTokens: 0, totalUSD: 0 };
 
   // Resolve concurrency from user input (default 1)