Add deterministic session restore support for Gemini CLI tabs

[danshapiro]

Problem

Gemini CLI tabs are not covered by the deterministic session restore work. In /home/user/code/freshell/.worktrees/deterministic-cli-session-identity-impl/server/terminal-registry.ts, Gemini has no resumeArgs, so Freshell treats it as a live-only provider. That means after a server restart, or after a tab has to be recreated, Freshell cannot reliably reconnect the browser tab to the same Gemini conversation.

This is a real user-facing gap: Freshell should not imply durable restore for a provider unless it has a proven provider-owned session identity and a deterministic way to resume that identity.

Current Evidence

• The current resilience pass proved durable behavior for OpenCode, Codex, and Claude Code through real-provider contracts or isolated smoke tests.
• Gemini launched in manual smoke, but did not produce a clean prompt/response/restore proof in that pass.
• The extension manifest at /home/user/code/freshell/extensions/gemini/freshell.json only provides command launch metadata.
• The fallback command spec in /home/user/code/freshell/server/terminal-registry.ts does not define Gemini session creation or resume arguments.
• Existing unit tests explicitly expect modeSupportsResume('gemini') to be false.

Required Work

1. Research Gemini CLI session persistence deterministically, using official docs/source and direct experiments.
2. Identify whether Gemini CLI exposes a canonical session id, transcript path, database row, or other provider-owned durable identity.
3. Identify the exact resume command, if one exists.
4. Add a real-provider contract that proves:
   • fresh tab launch reaches an interactive ready state,
   • a real prompt gets a response,
   • close/reopen reuses the live terminal while the server is still running,
   • server restart can resume the same Gemini conversation using only the provider-owned durable identity.
5. Implement Freshell resume support only if the proof exists.
6. Do not match by cwd, wall-clock time, tab title, or creation ordering.
7. If Gemini CLI does not support deterministic resume, make that explicit in code, tests, and user-facing behavior rather than silently pretending restore is supported.

Acceptance Criteria

• Gemini tabs either restore deterministically after server restart, or are clearly classified as non-resumable/live-only with intentional fresh-create behavior.
• The sidebar status for live Gemini tabs is accurate.
• New Gemini tabs appear in the sidebar when live.
• Restore failures emit structured server logs and client-visible errors where appropriate.
• There is no cwd/time/title heuristic for session identity.