feat(WARDEN-WP-0019): route secret-exec lanes to secrets-engine (route-primary, proxy fallback)

secrets-engine (SECRETS-WP-0003) shipped a native secret-exec front door
(`secrets-engine route/exec`, decision e6381a56) and asked ops-warden to route to it.
Bernd's call: route-primary, proxy-fallback — surface the secrets-engine exec as the
primary path for owned lanes, keep `warden access --exec` as a transparent fallback.

T1 — RouteEntry gains exec_owner/exec_command/pointer_command (+ has_native_exec),
screened for secret material like the other handoff fields. whynot-design-npm-publish
points its native exec at secrets-engine. `warden access` renders Primary (secrets-engine
exec) + Fallback (warden proxy); route/access JSON gain the fields and a native-exec-aware
next_action. Tests added; 217 pass, lint clean.

T2 — credential-routing.md adds secrets-engine as the secret-exec owner (route primary,
proxy fallback); SCOPE adds secrets-engine to Related Repos and records the npm lane as
production-exercised (@whynot/design@0.4.0); playbook leads with secrets-engine exec and
fixes the fallback one-liner (--field NPM_AUTH_TOKEN, --no-policy) per whynot-design.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/warden/routing/models.py

@@ -43,11 +43,23 @@ class RouteEntry:
     #              no identity yet), no caller-auth precheck (the point is to get one),
     #              run interactively as the caller; warden never captures the token.
     lane: str = "secret"
+    # Owner-native exec front door (WP-0019). When `exec_owner` is set, that subsystem
+    # (e.g. secrets-engine) provides the PRIMARY way to run a secret-backed command; the
+    # catalog routes to it and keeps ops-warden's own --fetch/--exec proxy as a transparent
+    # fallback (route-primary, proxy-fallback). Pointers/templates only — never a value.
+    exec_owner: Optional[str] = None         # subsystem owning the native exec (e.g. secrets-engine)
+    exec_command: Optional[str] = None       # e.g. "secrets-engine exec --catalog <id> -- <cmd>"
+    pointer_command: Optional[str] = None    # e.g. "secrets-engine route <id> --json"
 
     @property
     def is_active(self) -> bool:
         return self.status == "active"
 
+    @property
+    def has_native_exec(self) -> bool:
+        """True when an owner-native exec front door is the primary path for this lane."""
+        return bool(self.exec_owner and self.exec_command)
+
     @property
     def has_handoff(self) -> bool:
         """True when structured assist fields are present (advisory richness)."""