refactor: switched to upstream @mariozechner/pi-ai and removed unused packages

- Replaced local @oh-my-pi/pi-ai with upstream @mariozechner/pi-ai@0.37.4
- Added sessionId support to agent for provider caching (OpenAI Codex)
- Deleted packages/ai (now using upstream)
- Deleted packages/mom (unused)
- Deleted packages/web-ui (unused)

packages/coding-agent/examples/hooks/custom-compaction.ts

@@ -13,7 +13,7 @@
  *   omp --hook examples/hooks/custom-compaction.ts
  */
 
-import { complete, getModel } from "@oh-my-pi/pi-ai";
+import { complete, getModel } from "@mariozechner/pi-ai";
 import type { HookAPI } from "@oh-my-pi/pi-coding-agent";
 import { convertToLlm, serializeConversation } from "@oh-my-pi/pi-coding-agent";
 

packages/coding-agent/examples/hooks/handoff.ts

@@ -12,7 +12,7 @@
  * The generated prompt appears as a draft in the editor for review/editing.
  */
 
-import { complete, type Message } from "@oh-my-pi/pi-ai";
+import { complete, type Message } from "@mariozechner/pi-ai";
 import type { HookAPI, SessionEntry } from "@oh-my-pi/pi-coding-agent";
 import { BorderedLoader, convertToLlm, serializeConversation } from "@oh-my-pi/pi-coding-agent";
 

packages/coding-agent/examples/hooks/qna.ts

@@ -7,7 +7,7 @@
  * 3. Loads the result into the editor for user to fill in answers
  */
 
-import { complete, type UserMessage } from "@oh-my-pi/pi-ai";
+import { complete, type UserMessage } from "@mariozechner/pi-ai";
 import type { HookAPI } from "@oh-my-pi/pi-coding-agent";
 import { BorderedLoader } from "@oh-my-pi/pi-coding-agent";