fix(gemini): align headless session/auth flow with async CLI behavior

Why:
- Gemini does not expose a new session id synchronously.
  It emits the canonical id in the init stream event.
- Creating temporary ids in web mode introduced identity drift,
  extra mapping logic, and harder resume/debug behavior.
- Headless server runs often miss shell-inherited auth vars,
  while users configure Gemini through user-level env files.
- Gemini mirrors session artifacts across folders,
  which caused duplicate sync events and duplicate session rows.

What changed:
- Removed temporary Gemini ids for new sessions.
- New Gemini sessions are now created only after init provides session_id.
- Persisted cliSessionId from the discovered canonical id,
  keeping one identifier across stream, storage, and resume.
- Built Gemini spawn env from process env plus user-level fallback files:
  ~/.gemini/.env then ~/.env, honoring GEMINI_CLI_HOME.
- Added --skip-trust for headless runs,
  because web flows cannot answer interactive trust prompts.
- Improved terminal error mapping and rejection reasons,
  especially for auth exit code 41 with actionable context.
- Limited Gemini synchronization to tmp JSONL chat artifacts,
  and disabled duplicate watcher/index paths that mirror the same sessions.
- Added gemini-2.5-flash-lite to shared model constants.

Result:
- Gemini session identity is canonical and provider-consistent.
- Headless auth now matches practical Gemini CLI configuration patterns.
- Duplicate Gemini session indexing is reduced at the source.
- Operators get clearer, actionable failure messages.

server/modules/providers/services/sessions-watcher.service.ts

@@ -24,10 +24,12 @@ const PROVIDER_WATCH_PATHS: Array<{ provider: LLMProvider; rootPath: string }> =
     provider: 'codex',
     rootPath: path.join(os.homedir(), '.codex', 'sessions'),
   },
-  {
-    provider: 'gemini',
-    rootPath: path.join(os.homedir(), '.gemini', 'sessions'),
-  },
+  // {
+  //   provider: 'gemini',
+  //   rootPath: path.join(os.homedir(), '.gemini', 'sessions'),
+  // },
+  // Keep `sessions/` watcher disabled: Gemini also mirrors artifacts there,
+  // which causes duplicate synchronization events.
   {
     provider: 'gemini',
     rootPath: path.join(os.homedir(), '.gemini', 'tmp'),