Session storage grows unboundedly — add `opencode session prune` command and auto-cleanup config

[vsixer]

Problem

opencode stores all session history, messages, file parts, and diffs in ~/.local/share/opencode/storage/ with no automatic cleanup and no CLI command to reclaim disk space.
After ~5 months of regular use on a single project, my storage has grown to:

~/.local/share/opencode/
├── storage/
│   ├── message/          497 MB   (conversation messages)
│   ├── part/             966 MB   (30,122 files — file content snapshots)
│   ├── session_diff/     989 MB   (session state diffs)
│   └── session/          5.3 MB   (1,324 session index files)
└── tool-output/           52 MB   (Playwright, shell outputs)
Total: ~5.1 GB

Growth rate: ~500 MB/month. At this pace, it will reach 6+ GB within a year for a single project.

What I'd expect

1. CLI command: opencode session prune

# Dry run — show what would be deleted
opencode session prune --older-than 30d --dry-run
# Delete sessions older than 30 days and reclaim disk space
opencode session prune --older-than 30d
# Show current storage usage by category
opencode session stats

2. Config option for automatic cleanup

"session": {
  "retention_days": 30,  // auto-prune sessions older than N days
  "max_storage_mb": 2048 // or cap by total size
}

3. What should be cleaned up

When a session is deleted, all associated data should be removed:

• storage/session/{project_id}/ses_*.json
• storage/message/ entries linked to that session
• storage/part/ entries linked to those messages (the biggest contributor at ~1 GB)
• storage/session_diff/ses_*.json
• tool-output/ entries linked to that session

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• Feature requests must use the [FEATURE]: prefix in the title (e.g. [FEATURE]: Session storage grows unboundedly...)
• The feature request template requires checking the verification checkbox confirming the feature hasn't been suggested before

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

---

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Session Lifecycle Management — unified storage reclamation, auto-archival, and child session cleanup #16101: Session Lifecycle Management — covers the same unbounded storage growth problem, proposes opencode session prune, auto-cleanup config, opencode session stats, and SQLite VACUUM in essentially the same scope

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

This issue has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new issue that follows our issue templates.