fix(security): active guard enforcement, CEO A2A target check, notification expiry (#595)

* fix(security): guard goes active; CEO A2A respects no-comms roles; ack notifications expire

ROBOCO_GUARD_PASSIVE_MODE defaults to false in both compose files — the
deferred post-calibration flip; fail_secure stays off and the env override
remains the rollback. can_a2a_direct no longer short-circuits the CEO past
the no-comms set (auditor/pr_reviewer/prompter/secretary), now canonical
in foundation.policy.communications.NO_COMMS_ROLES and shared with the
content-actions gate; the A2A service refuses at conversation creation
instead of silently suppressing the wake. Ack-required notifications get
expires_at stamped from ROBOCO_NOTIFICATION_ACK_TTL_HOURS (default 48,
0 disables), so the re-escalation sweeper's expires_at query matches rows
for the first time.

* refactor(notification): extract _ack_and_expiry — xenon rank back under B

The expires_at stamping pushed _create_notification_with_session to
rank C; the requires_ack + expiry derivation moves into a helper with
the same semantics and comments.

* test(conftest): dispose the global DB engine after every test

Production code reaching get_db_context()/get_engine() lazily creates the
process-global engine bound to the current event loop; with per-test
function-scoped loops, any later test touching the global path inherits a
dead-loop engine and dies with 'Future attached to a different loop' —
the order-dependent class that has been wandering the suite (cloud_auth
login, metrics, tasks-routes, full-lifecycle) whenever collection order
shifts. An autouse fixture now close_db()s after every test, keeping the
global path loop-local; no-op when untouched.

---------

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

.env.example

@@ -235,8 +235,10 @@ ROBOCO_DB_NETWORK_ISOLATED=true
 # fastapi-guard HTTP security layer — v0.16.0
 # =============================================================================
 # Master switch + calibration knobs. Off by default; the NAS build compose
-# arms it in PASSIVE (log-only) mode for false-positive review before
-# enforcing. Not exposed on the panel's Feature Flags card (still calibrating).
+# arms it in ACTIVE enforcement (passive/log-only calibration reviewed
+# clean — see docs/rag/architecture/http-security-guard.md). Not exposed on
+# the panel's Feature Flags card (compose/env-coupled, like
+# ROBOCO_DB_NETWORK_ISOLATED).
 # ROBOCO_GUARD_ENABLED=false
 # Detect-and-log without blocking.
 # ROBOCO_GUARD_PASSIVE_MODE=true