F-antigravity-sessions: Session Discovery & Scan

[bastien-gallay]

Issue #145 — Antigravity Session Discovery & Scan

Extend FsScanner to scan Antigravity sessions and group them by project.

Background

Claude sessions are organized on disk under ~/.claude/projects/ using encoded
project paths. Antigravity sessions are stored in flat directories named by
conversation UUID under ~/.gemini/antigravity-cli/brain/.

Requirements

1. Modify FsScanner (in crates/scan) to scan
   ~/.gemini/antigravity-cli/brain/.
2. For each subdirectory (conversation UUID):
   • Locate .system_generated/logs/transcript.jsonl.
   • Use the Antigravity transcript parser (chore: strip issue-number refs from code comments; document the rule #144) to extract CWD and metadata.
   • Build a SessionRecord where session_id is the conversation UUID.
3. Group these records by the derived CWD alongside existing Claude records
   in the sidebar.
4. Integrate the scan with ScanCache so that transcript parsing is
   incremental (based on transcript.jsonl mtime/size), preventing slow cold
   starts.

Tests

• Scan a mock brain/ directory containing Antigravity conversation structures.
• Group Antigravity sessions correctly by derived project path.
• Cache hits/misses behave correctly when files are touched or modified.

[bastien-gallay]

Refactor prerequisites: #170 (E — the agy scanner is a second walk source over the shared cache/watch) and #169 (A3 Sessions registry, for the second session kind).