[LHagfoss]

What

RustCode compacted a Claude Code thread on its own at 154k of a 1M window, because DEFAULT_AUTO_COMPACT.mode was "size", and the context popover then said the harness "compacts on its own". Now:

• The default mode is off. A stored choice (size or cache, object or old string form) is still read back unchanged.
• When RustCode starts a compaction, the divider reads "Compacting context (auto, over 150k)…" (or "…before the cache expires…"), and it and the "Compacted automatically" row link to the Auto-compact setting.
• The context popover for a harness that reports totals only says "RustCode compacts it above N" when the setting applies, and the harness's own behavior only otherwise.
• Settings says the setting is off by default; turning it on is one change in the existing Auto-compact row.

Changelog

RustCode no longer compacts threads on its own unless you turn on Auto-compact in Settings; when it does, the divider says so and links to the setting.

Testing

bun run check in apps/desktop (typecheck, biome, tests, build). New tests in autoCompact.test.ts (default off, stored choice kept, divider text) and autoCompact.store.test.ts (a fresh install does not compact; the running divider carries the reason and the setting link). Rule tests now set the size rule on explicitly.

Closes #3439