supply-chain: gate our own deps, ship the verdict signed

dep-scan.sh gates go/npm/cargo on push and bakes an attested posture into the
release — embedded in the server, signed into the manifest. Reasoning and the
two Moby exceptions are in SECURITY.md.

(posture-builder runs rustup; bookworm's cargo is too old for cargo-audit.)

SECURITY.md

@@ -54,6 +54,29 @@ Before any package install: the agent fetches the expected SHA-256 from the serv
 
 ---
 
+## Our Own Dependencies
+
+RedFlag holds itself to the standard it enforces on the fleet. Every push runs
+dependency vulnerability scanning in CI: `govulncheck` (reachability-based) on the Go
+server and agent, `npm audit` on the web tree, `cargo audit` on the Rust helper. A
+reachable vulnerability that isn't explicitly accepted fails the build — the same
+fail-closed posture the binary takes at runtime.
+
+Some findings have no fix to take. RedFlag links the Docker engine library as a
+*client* (for container scanning) and inherits daemon-side Moby advisories that carry
+no patched version. We don't bury those: each one is a documented entry in a
+machine-readable exception register, every entry naming the advisory and the reason
+it's accepted. That register is the source of truth — it's surfaced **in the app**, so
+RedFlag's own residual exposure is visible the same way fleet exposure is, and can't
+quietly rot in a doc nobody re-reads. When an upstream fix ships, the dependency is
+bumped and the entry is removed; CI warns on an exception that no longer applies.
+
+The build substrate itself is recorded on every run (toolchain and engine versions) so
+"what built this" is never a mystery. Enforcing a minimum-patched floor on that
+substrate is the next layer.
+
+---
+
 ## Visibility
 
 **Security Health** is surfaced as a dashboard panel on each agent — signing status, nonce protection, machine binding violations, command validation — so the posture is visible without digging through logs. All operations are logged with full context, sanitized against log injection (ANSI stripping, control character replacement, field truncation) with the content preserved.