Gemini CLI backfill rescans every project directory for every transcript #176

Gemini CLI transcripts don't record the working directory, so for each transcript the parser lists every project under ~/.gemini/tmp, every chats/ folder, and opens matching files to find the project root. A backfill over many sessions in many projects does that full scan once per session.

Applies to the agent support in #149.

Files:

• crates/trodden/src/harness/gemini.rs

Fix idea: build the session-to-project map once per backfill, or pass the transcript path to the parser, since its parent directory already names the project.