trivy-fs-scan: enforce by default; 'false' is the emergency escape (#5)

Feedback from the first rollout: a default-off gate is the wrong way
around — the default must be active and enforced, with the opt-out as an
explicit, documented per-invocation emergency flag (same posture as the
image-scan gate).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P5NjXQ3jmyVjimHQxnyKhg
Reviewed-on: #5
Co-authored-by: peet <peet@noctrl.eu>

README.md

@@ -123,9 +123,10 @@ from repo secrets; no-op when the secret is unset (build stays unsigned).
 ### Trivy Filesystem Scan
 
 Scans the checked-out workspace for vulnerable dependencies (lockfile-based) —
-for repos that build artifacts (APKs) rather than images. Non-blocking by
-default (`enforce: "false"`); requires committed Gradle lockfiles and the
-trivy binary on the runner. Pin the job to `linux,build,amd64`.
+for repos that build artifacts (APKs) rather than images. Enforcing by default
+(fails on fixable HIGH/CRITICAL); `enforce: "false"` is a temporary emergency
+escape only. Requires committed Gradle lockfiles and the trivy binary on the
+runner. Pin the job to `linux,build,amd64`.
 
 **Location:** [`./trivy-fs-scan`](./trivy-fs-scan)