feat(sessions): automatic retention policy and cleanup for session metadata on disk

[euxaristia]

Summary

Zero currently persists session metadata and event logs to disk (~/.local/share/zero/sessions/ on Unix, %LOCALAPPDATA%\zero\sessions\ on Windows), but never prunes or deletes old session directories. Over extended usage, session logs and checkpoint blobs accumulate indefinitely without an automatic retention policy or CLI cleanup mechanism.

Motivation

While in-memory prompt compaction (internal/agent/compaction.go) and tool body pruning (internal/agent/prune.go) manage context token pressure, on-disk storage grows monotonically:

• Every session generates a separate directory containing metadata.json, events.jsonl, and optional checkpoint blobs.
• internal/sessions/store.go provides listing and appending, but no cleanup lifecycle or deletion API.
• Users currently have to manually inspect and delete old session directories.

Proposed Solution

1. Configurable retention policy: Introduce configuration settings in config.json (e.g. sessions.retention_days or sessions.max_count) to automatically prune stale, non-bookmarked session directories.
2. CLI cleanup command: Add zero sessions clean (or zero sessions prune --older-than <duration>) to allow users to trigger metadata sweeps manually.
3. Blob & checkpoint garbage collection: Prune unreferenced rewind blobs when sessions are removed.
4. Safety guards: Ensure active sessions (holding session.lock) and explicitly saved/named sessions are protected from automated pruning.