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.

shared/modelConstants.js

@@ -84,6 +84,7 @@ export const GEMINI_MODELS = {
     { value: "gemini-2.5-flash", label: "Gemini 2.5 Flash" },
     { value: "gemini-2.5-pro", label: "Gemini 2.5 Pro" },
     { value: "gemini-2.0-flash-lite", label: "Gemini 2.0 Flash Lite" },
+    { value: "gemini-2.5-flash-lite", label: "Gemini 2.5 Flash Lite" },
     { value: "gemini-2.0-flash", label: "Gemini 2.0 Flash" },
     { value: "gemini-2.0-pro-exp", label: "Gemini 2.0 Pro Experimental" },
     {