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/protocol.ts

@@ -16,12 +16,6 @@ export type SidecarCommand =
   | { type: 'claude:spawn-streaming'; id: string; params: ClaudeSpawnStreamingParams }
   | { type: 'claude:kill'; id: string; sessionKey: string }
   | { type: 'claude:clear-session'; id: string; sessionKey: string }
-  // Pi
-  | { type: 'pi:spawn'; id: string; params: PiSpawnParams }
-  | { type: 'pi:prompt'; id: string; sessionId: string; prompt: string; requestId: string }
-  | { type: 'pi:abort'; id: string; sessionId: string; requestId: string }
-  | { type: 'pi:kill'; id: string; sessionId: string }
-  | { type: 'pi:set-thinking'; id: string; sessionId: string; level: string }
   // VNC
   | { type: 'vnc:start'; id: string; params: VncStartParams }
   | { type: 'vnc:stop'; id: string; email: string }
@@ -40,11 +34,6 @@ export type SidecarEvent =
   | { type: 'claude:error'; id: string; error: string }
   | { type: 'claude:killed'; id: string }
   | { type: 'claude:session-cleared'; id: string }
-  // Pi
-  | { type: 'pi:spawned'; id: string; sessionId: string }
-  | { type: 'pi:event'; sessionId: string; event: PiEvent }
-  | { type: 'pi:error'; id: string; error: string }
-  | { type: 'pi:killed'; id: string }
   // VNC
   | { type: 'vnc:started'; id: string; port: number; display: number }
   | { type: 'vnc:stopped'; id: string }
@@ -62,16 +51,6 @@ export type ClaudeState = {
   claudeSessions: Record<string, string>; // sessionKey → Claude Code session_id
 };
 
-export type PiSessionInfo = {
-  sessionId: string;
-  email: string;
-  userId: number;
-  model: string;
-  cwd: string;
-  pid: number;
-  alive: boolean;
-};
-
 // ── Param types ──
 
 export type ClaudeSpawnParams = {
@@ -102,17 +81,6 @@ export type ClaudeCodeResult = {
   cost: MessageCost;
 };
 
-export type PiSpawnParams = {
-  sessionId: string;
-  email: string;
-  userId: number;
-  username: string;
-  role: string;
-  cwd: string;
-  model: string;
-  sessionFile?: string;
-};
-
 // ── VNC types ──
 
 export type VncStartParams = {