Gemini agent CLI support

[gadflysu]

Goal

Add aps support for Google Gemini CLI sessions.

Requirements

• Discover Gemini CLI sessions from the authoritative local storage format.
• Show Gemini sessions in list mode and interactive picker mode with normalized metadata.
• Filter Gemini sessions by current working directory using the existing path filter behavior.
• Preview Gemini session info using the existing preview boundary.
• Launch the selected Gemini session through the Gemini CLI using the existing launcher boundary.
• Include Gemini in the combined all-agents mode once the loader and launcher are implemented.

Known Context

• docs/agent/notes-dev-guide-new-client.md describes the expected steps for adding a new client.
• Gemini CLI storage, session metadata, title strategy, timestamp units, and resume command behavior still need investigation before implementation.

Research Questions

• What are the current Gemini CLI session storage paths and schema on this machine and in current source/docs?
• What fields are available for session id, working directory, title/summary, timestamp, and message/turn count?
• How should archived, deleted, generated, or subagent-like records be filtered if Gemini stores them?
• What exact CLI command should launcher exec for resuming a selected Gemini session?

Planning

Solution details are not yet complete. The executor should investigate Gemini storage/runtime behavior, then create docs/agent/plan-issue-N-*.md, link it from this issue body, and commit the plan before implementation.

Acceptance Criteria

• Gemini sessions can be loaded, filtered by current working directory, listed, previewed, and launched.
• Gemini follows the existing source/display/picker/preview/launcher boundaries.
• Unit tests cover storage parsing, empty/missing storage behavior, timestamp normalization, and launcher command construction.
• go test ./... passes.