gemini: wire session resume on workspace restart

[mafredri]

Problem

Gemini CLI supports gemini --resume "latest" for resuming the most recent session, and stores session JSON files under ~/.gemini/tmp/<project_hash>/chats/. The module's start.sh always starts a fresh session with no resume logic. It also doesn't pass --type gemini to agentapi.

The module currently passes the task prompt via --prompt-interactive "$PROMPT". On resume, this must be skipped.

Desired outcome

• On cold start: Gemini starts normally with the task prompt.
• On warm start (prior sessions exist): Gemini resumes the latest session. The task prompt is not re-sent.
• --type gemini is passed to agentapi server.
• After feat(coder/modules/agentapi): add state persistence #736 lands: enable_state_persistence = true.

Key upstream behaviors

• Critical: --resume "latest" causes a dead loop when no sessions exist (google-gemini/gemini-cli#15892). The module MUST check for existing sessions before passing --resume. Session files live under ~/.gemini/tmp/*/chats/*.json.
• Session files can grow very large (400MB+ reported in google-gemini/gemini-cli#15292) for long-running tasks.

Version bump

minor

Refs #696