fix: resolve Gemini CLI 0.37.0 session capture failures (#1664) (#1692)

Three root causes prevented Gemini sessions from persisting prompts,
observations, and summaries:

1. BeforeAgent was mapped to user-message (display-only) instead of
   session-init (which initialises the session and starts the SDK agent).

2. The transcript parser expected Claude Code JSONL (type: "assistant")
   but Gemini CLI 0.37.0 writes a JSON document with a messages array
   where assistant entries carry type: "gemini". extractLastMessage now
   detects the format and routes to the correct parser, preserving
   full backward compatibility with Claude Code JSONL transcripts.

3. The summarize handler omitted platformSource from the
   /api/sessions/summarize request body, causing sessions to be recorded
   without the gemini-cli source tag.

Co-authored-by: Claude <noreply@anthropic.com>

src/cli/adapters/gemini-cli.ts

@@ -13,7 +13,7 @@ import type { PlatformAdapter } from '../types.js';
  *   Notification  → observation (system events like ToolPermission)
  *
  * Agent:
- *   BeforeAgent   → user-message (captures user prompt)
+ *   BeforeAgent   → session-init (initializes session, captures user prompt)
  *   AfterAgent    → observation  (full agent response)
  *
  * Tool: