Add SecurityPractice.md, Security Genome record, and deny-escalation lockout

Design doc for hardening qonto-assistant before deployment to
railiance01: this is the first fleet service that must be
internet-reachable (external harness clients, not just in-cluster
jobs) while holding a real bank credential. Covers identity (key-cape
in place of the interim bearer token), authorization (finance.qonto.read
in flex-auth + tenant-engine capability roles instead of the
hardcoded default_tenant_id), network exposure (facade-only internet
address), isolation profile, and a Kings Guard mapping (the existing
audit stream is already Immune-Observation-shaped; nothing to rebuild
later).

Ships one concrete, dependency-free piece of that design now:
DenyEscalationTracker locks out an actor who repeatedly triggers
arg_constraint/credential_exfil denies within a short window, closing
the gap where a probing client could retry indefinitely at whatever
rate the existing rate limiter otherwise allows. Wired through
CapabilityService, on by default, configurable via
QONTO_DENY_ESCALATION_* env vars. Ordinary denies (authz_denied,
tenant_scope) never count toward it.

Also adds specs/security-genome.yaml (kings-guard's genome-record
shape, populated now so no rework is needed once a consumer exists).

Verified: pytest -> 39 passed (8 new); REST and MCP smoke scripts both
pass against fixtures; compileall clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/qonto_assistant/app.py

@@ -21,6 +21,7 @@ from qonto_assistant.mcp_server import create_mcp_server
 from qonto_assistant.policy import PolicyEngine
 from qonto_assistant.qonto_client import FixtureQontoClient, QontoClient
 from qonto_assistant.rate_limits import ConcurrencyLimiter, RateLimiter
+from qonto_assistant.security_watch import DenyEscalationTracker
 from qonto_assistant.service import CapabilityService
 
 
@@ -171,6 +172,15 @@ def _build_service(
         ),
         concurrency_limiter=concurrency_limiter
         or ConcurrencyLimiter(limit=settings.max_concurrency),
+        deny_escalation_tracker=(
+            DenyEscalationTracker(
+                threshold=settings.deny_escalation_threshold,
+                window_seconds=settings.deny_escalation_window_seconds,
+                lockout_seconds=settings.deny_escalation_lockout_seconds,
+            )
+            if settings.deny_escalation_enabled
+            else None
+        ),
     )