Refactor entry-side deletion refusal to execute in-transaction via
`_write_guards_reject(conn, sid)` (#123583), per maintainer review:
- Underlying `delete_session` and `delete_sessions` now accept an opt-in
kwarg `exclude_active_write_guards=True` running inside `_do` write
transaction, eliminating the race condition where a turn acquires the lease
between check and delete.
- Raises `SessionActiveWriteGuardError` when refusing single delete, leaving
the row untouched; `delete_sessions` atomically skips active rows.
- Checks both active turn leases and compression locks via the existing
reclaim-aware `_write_guards_reject` helper.
- Covers all user-facing delete sinks:
* Web `DELETE /api/sessions/{id}` -> 409 Conflict
* Web `POST /api/sessions/bulk-delete` -> skips active rows
* Web / CLI `prune` -> passes `exclude_active_write_guards=True` so lineage
parents of active conversations are not pruned
* API Server `DELETE /api/sessions/{id}` -> 409 session_active_turn
* CLI `hermes sessions delete` & `export --delete-after-verified` -> exits 1
* CLI browse picker -> refuses active delete
* TUI Gateway `session.delete` -> 4023 error
- Conforms to rubric with 2 targeted invariant tests in
`tests/hermes_state/test_delete_session_write_guards.py`.
- Updates user guide and web dashboard docs for 409 / exit 1.
(cherry picked from commit 2c037a7a79dc211b49bacc72e3140951ccf900cf)
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