feat(audit): persistent audit log of agent-initiated privileged actions

Adds a durable, operator-visible audit trail of privileged operations
hive-c0re performs on behalf of an agent — the ones that cross the
agent/operator trust boundary. First entry: infra-container restarts via
the infra_admin-gated `restart` tool, which until now were recorded only
as a hive-priv journal trace.

Backend:
- new `audit_log` module: sqlite-backed store (audit_log.sqlite, same dir
  as build_logs) with schema (ts/agent/action/target/outcome/detail),
  best-effort `record`, `list_recent` (clamped 500), 90-day `vacuum`, and
  a process-singleton handle mirroring build_logs.
- Coordinator opens + installs the handle; main spawns the hourly vacuum.
- agent_server::handle_restart_infra records every attempt (ok, error, and
  capability-denied) via the global handle — best-effort, never fails the
  underlying action.
- dashboard: `GET /api/audit-log` returns recent entries as JSON.

Scope is deliberately agent-initiated privileged actions only (not every
PrivRequest — token writes + nspawn edits are constant lifecycle noise).
Extensible: future agent-initiated priv ops record via the same handle.

Unit tests cover record/list ordering, the 500 clamp, and retention vacuum.

The dashboard *surface* (an AUDIT view consuming /api/audit-log) is a
frontend follow-up coordinated with iris.

hive-c0re/src/main.rs

@@ -397,6 +397,9 @@ async fn cmd_serve(
     // build_logs.sqlite vacuum: c0re-side (single db). Failures kept
     // 30d, successes 24h — see `build_logs::vacuum` for the rule.
     hive_c0re::build_logs::spawn_vacuum(&coord);
+    // audit_log.sqlite vacuum: agent-initiated privileged-action trail,
+    // 90d retention — see `audit_log::vacuum`.
+    hive_c0re::audit_log::spawn_vacuum(&coord);
     // Container crash watcher: emits HelperEvent::ContainerCrash
     // when a previously-running container goes away without an
     // operator-initiated transient state.