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/sidecar-registry.ts

@@ -8,7 +8,6 @@ import type {
   ClaudeSpawnParams,
   ClaudeSpawnStreamingParams,
   ClaudeCodeResult,
-  PiSpawnParams,
   PtyCommand,
   PtyEvent,
   VncStartParams,
@@ -351,42 +350,6 @@ export function onClaudeEvent(handler: (sessionKey: string, event: PiEvent) => v
   });
 }
 
-// ── Pi ──
-
-export async function spawnPi(params: PiSpawnParams): Promise<void> {
-  const res = await sendCommand('pi', { type: 'pi:spawn', id: nextId(), params });
-  if (res.type === 'pi:spawned') return;
-  if (res.type === 'pi:error') throw new Error(res.error);
-  throw new Error('Unexpected response');
-}
-
-export function sendPiPrompt(sessionId: string, prompt: string, requestId: string): void {
-  sendFire('pi', { type: 'pi:prompt', id: nextId(), sessionId, prompt, requestId });
-}
-
-export function abortPi(sessionId: string, requestId: string): void {
-  sendFire('pi', { type: 'pi:abort', id: nextId(), sessionId, requestId });
-}
-
-export function killPi(sessionId: string): void {
-  sendFire('pi', { type: 'pi:kill', id: nextId(), sessionId });
-}
-
-export function setPiThinking(sessionId: string, level: string): void {
-  sendFire('pi', { type: 'pi:set-thinking', id: nextId(), sessionId, level });
-}
-
-export function onPiEvent(handler: (sessionId: string, event: PiEvent) => void): () => void {
-  return on('pi:event', (msg) => {
-    if (msg.type === 'pi:event') {
-      handler(
-        (msg as SidecarEvent & { type: 'pi:event' }).sessionId,
-        (msg as SidecarEvent & { type: 'pi:event' }).event,
-      );
-    }
-  });
-}
-
 // ── Terminal (PTY sidecar) ──
 
 export function sendPtyCommand(cmd: PtyCommand): void {