[P2] Management affordances: retention and cleanup story

[jsirish]

From docs/AUDIT-v0.4.0.md (P2, item 9).

Everything grows without bound: logs/ (pruning explicitly forbidden by consolidate), buffer/archive/, and .capture-errors (onboard says "clear the file once resolved" but provides no affordance). There is no status surface beyond the SessionStart block.

Proposal (keep it infrastructure-free, per the plugin's identity):

1. README "Housekeeping" section: what is safe to delete (archived buffers older than the last consolidation pass, a resolved .capture-errors), what is not (logs/ are the evidence trail).
2. Teach the handoff skill to clear .capture-errors after surfacing its contents in a session log (the breadcrumbs are consumed once distilled).
3. Optional: an archive-rotation guideline (e.g. archived buffers > 90 days old can go) documented rather than automated.