Add session prune command — scoped cleanup by author

[efenocchi]

Context

Session JSONL files are now read-only in the VFS (EPERM on write/rm/mv). Users need a way to clean up old sessions without bypassing protections.

Proposal

Add a CLI command for session cleanup, scoped to the current user's sessions only:

hivemind sessions prune                          # list pruneable sessions
hivemind sessions prune --before 2026-04-01      # delete sessions older than date
hivemind sessions prune --session-id abc-123      # delete specific session
hivemind sessions prune --all                     # delete all own sessions

Rules

• Only delete sessions where author matches the logged-in user from credentials
• Cannot delete other users' sessions
• Deletes both the session rows (sessions table) and the corresponding summary (memory table)
• Requires confirmation unless --yes is passed

Related

PR #33 added EPERM guards on session files. This command provides the controlled alternative.