fix(sessions): preserve recently active sessions during pruning (salvage of #71043 by @Frowtek)

[teknium1]

Summary

Auto-prune no longer deletes a recently-active conversation just because it was created long ago.

Root cause: prune_sessions / list_prune_candidates converted older_than_days to started_before on started_at. An ended, long-lived conversation used today but created >90d ago was deleted — and VACUUM makes recovery impractical.

Changes

• hermes_state.py: retention age is now inactivity — COALESCE(MAX(messages.timestamp), started_at).
• session_filters.py: new last_active_before/after; --older-than/--newer-than bound activity, --before/--after keep bounding start time.
• CLI / dashboard / docs surface last_active (dashboard keeps oldest/newest_started_at for compat).

Note

Deliberate CLI semantics change: --older-than + --before become independent axes (was: tighter-bound-wins). Documented; test replaced accordingly. Conceptually overlaps #70429's indexed last_active column — coordinate if both land; prune runs at most once per interval so the correlated subquery cost is acceptable.

Validation

Real-SQLite regression tests across 5 files: 609 passed.

Salvage of #71043 — @Frowtek's commit cherry-picked with authorship preserved.

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[github-actions]

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