compact-plus algorithm for /compact

What it does

Brings the approach of compact-plus (https://github.com/u-ichi/compact-plus) to /compact: before compaction the transcript is backed up and a state file is written; after it, the state file is injected back.

Source

ganja-code-za2o (the compact-plus algorithm for /compact).

The bead is closed when this issue is published, with its external reference pointing here (#38).

File and line references in this issue are to main as of 75a7a92.

Against the three criteria

• Direction (README): The README says nothing about compaction.
• Today's terminal users: Compaction today is a summary prompt (crates/ganja-core/src/session.rs:232-279). ganja already has the PreCompact and SessionStart hook events (crates/ganja-core/src/hook.rs:122-140).
• Evidence: The bead holds only the URL, so which part would come into ganja is undecided. Product text is ganja-original by the 2026-09-02 ruling, and MIT text brought in would need a THIRD_PARTY_NOTICES.md entry.

Depends on

Related to #89 (a related edge, not blocking).

Verdict

• Verdict: park
• Reopens: at the next landing that touches compaction (compact_if_needed, the summary prompt).
• Exception to the probe rule: no.
• Date: 2026-09-19
• Verdict ticket: #42

Theme: #56

Start with crates/ganja-core/src/session.rs:232-279 and crates/ganja-core/src/hook.rs:122-140 to understand current compaction and hook events, then review the linked compact-plus source. The issue is explicitly parked because the imported behavior and product scope are undecided; completion criteria are not defined until that scope is settled.