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>

ecosystem.config.cjs

@@ -12,12 +12,6 @@ module.exports = {
       args: 'run src/servers/sidecar/claude/index.ts',
       watch: false,
     },
-    {
-      name: 'officer-pi',
-      script: 'bun',
-      args: 'run src/servers/sidecar/pi/index.ts',
-      watch: false,
-    },
     {
       name: 'officer-email',
       script: 'bun',