fix(sessions): preserve recently active sessions during pruning

[Frowtek]

What does this PR do?

Prevents session retention from deleting long-lived conversations that were active recently.

prune_sessions(older_than_days=...) previously compared the retention cutoff against sessions.started_at. As a result, an ended conversation created more than 90 days ago but used today was silently deleted at CLI/gateway startup when sessions.auto_prune was enabled. This removed the session row, its messages, and transcript files; the normal follow-up VACUUM could make recovery impractical.

Retention age now uses the latest persisted message timestamp, falling back to started_at for empty sessions. Explicit --before and --after filters continue to target session creation time, while relative --older-than and --newer-than filters target activity.

The preview, confirmation output, dashboard response, documentation, and deletion path use the same activity contract.

Related Issue

No issue filed.

Related but non-duplicate work:

• Fix abandoned session cleanup #38327 handles abandoned sessions whose ended_at remains NULL.
• feat(state): indexed sessions.last_active for recents ordering + legacy trigram opt-out #70429 adds indexed recency ordering but does not change retention deletion semantics.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• Updated hermes_state.py so age-based preview and pruning use latest message activity.
• Kept explicit session-start filters separate from relative activity filters.
• Updated CLI confirmation and dashboard dry-run output to expose last_active.
• Added real SQLite regression coverage for DB rows, messages, and transcript preservation.
• Updated CLI/filter tests and session-retention documentation.

How to Test

1. On main, create an ended session whose started_at is 100 days old, append a message with the current timestamp, and run:
   maybe_auto_prune_and_vacuum(retention_days=90, vacuum=False, sessions_dir=...)
2. Observe that main@7cd48733d returns pruned: 1 and deletes the session, messages, and transcript.
3. Repeat on this branch and observe pruned: 0; the session, message, and transcript remain.
4. Run:
   .\.venv\Scripts\python.exe -m pytest -o "addopts=-m 'not integration'" -p no:timeout tests/test_hermes_state.py tests/hermes_cli/test_session_filters.py tests/hermes_cli/test_sessions_delete.py tests/hermes_cli/test_sessions_export_md_cli.py tests/hermes_cli/test_dashboard_admin_endpoints.py -q
   Result: 574 passed.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: my locally

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — website/docs/user-guide/sessions.md
• I've updated cli-config.yaml.example if I added/changed config keys — N/A, no config keys changed
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide
• I've updated tool descriptions/schemas if I changed tool behavior — N/A

Screenshots / Logs

Current main:

{"started_days_ago":100,"latest_message_age_seconds":0,"maintenance":{"skipped":false,"pruned":1,"vacuumed":false},"session_after":null,"transcript_exists":false}

This branch:

{"started_days_ago":100,"latest_message_age_seconds":0,"maintenance":{"skipped":false,"pruned":0,"vacuumed":false},"session_survived":true,"messages":1,"transcript_exists":true}

Regression suite:

574 passed in 74.09s

[teknium1]

Merged via #72358 with your commit cherry-picked onto current main — authorship preserved. Thanks for the prune-by-inactivity fix.