fix(docs,code): ARCH-004 + SEC-003-K8S + ARCH-003 — marketing claims now match code truth

Sprint 4 unified-master-audit closure. Three claim-truth-alignment
findings whose README edits land on shared lines, bundled into one
commit.

ARCH-004 — 'full REST API exposed as MCP tools' overclaim:
  Pre-fix the README said 'the full REST API is exposed as MCP
  tools'; the actual MCP coverage is 162 tools / 220 routes
  (~74%). The remaining gap is intentional: protocol-conformance
  endpoints (ACME/SCEP/EST/OCSP/CRL), browser-only auth flow,
  health/ready, and streaming/binary downloads — categories that
  don't fit the request-response JSON tool shape.

  Fix:
    - README L78 qualified to 'the bulk of the REST API surface'
      with explicit numbers + pointer to the new coverage doc.
    - New docs/reference/mcp-coverage.md publishes the exclusion
      categories with rationale + the canonical commands to
      re-derive route + tool counts.
    - New scripts/ci-guards/mcp-coverage-parity.sh fails the build
      if the tool count drops below (routes − exclusions − 40-slack),
      so a future regression that drops 50+ tools surfaces in CI.
      Verified locally: clean at 162 tools / 220 routes / 37
      intentional exclusions.

SEC-003-K8S — Kubernetes Secrets connector is a runtime stub:
  Pre-fix README L67 marketed 'fifteen native target connectors'
  with Kubernetes Secrets in the list, but realK8sClient's CRUD
  methods returned 'real Kubernetes client not implemented' in
  production. Per the audit's option (b) recommendation: downgrade
  marketing + runtime-guard the stub.

  Fix:
    - README L12 + L67: 'fourteen production-ready native deployment-
      target connectors plus Kubernetes Secrets (preview)'.
    - k8ssecret.New() now refuses to construct unless
      CERTCTL_K8SSECRET_PREVIEW_ACK=true is set, mirroring the
      SEC-H3 ACK pattern. NewWithClient path (test injection)
      unchanged.
    - docs/reference/connectors/index.md moves Kubernetes Secrets
      out of the canonical fourteen-target list into a new 'Preview
      connectors' subsection.
    - Regression tests in k8ssecret_test.go pin the new gate
      (rejects without ACK, accepts with ACK, still rejects nil
      config even with ACK).

ARCH-003 — CERTCTL_KEYGEN_MODE=server breaks the blanket claim:
  Pre-fix README L12 + L82 said 'private keys stay on your
  infrastructure' and 'never touch the control plane' as blanket
  promises. Flipping CERTCTL_KEYGEN_MODE=server makes the control
  plane mint keys in process memory — breaking the claim — and
  the only signal was a boot-time slog WARN. An operator who set
  the flag and didn't read logs ran in silent contradiction to the
  marketed posture.

  Fix:
    - config.Validate() refuses to accept KeygenMode='server'
      unless DemoModeAck=true (mirroring SEC-H3). Production
      deploys (the default Mode='agent' path) are unaffected.
    - README L12 + L82 qualified: 'In agent-mode (the default),
      private keys ...; a demo-only CERTCTL_KEYGEN_MODE=server
      flag mints keys server-side, refuses to start without an
      explicit CERTCTL_DEMO_MODE_ACK=true acknowledgement.'
    - Regression tests for the new Validate gate land in
      config_test.go (note: gate tests landed in the ARCH-002
      commit because of contiguous-hunk constraint at the bottom
      of the file).

Closes ARCH-004, SEC-003-K8S, ARCH-003.

internal/config/config.go

@@ -1044,6 +1044,27 @@ func (c *Config) Validate() error {
 	if !validKeygenModes[c.Keygen.Mode] {
 		return fmt.Errorf("invalid keygen mode: %s (must be 'agent' or 'server')", c.Keygen.Mode)
 	}
+	// ARCH-003 closure (Sprint 4, 2026-05-16). README L12 + L82 say
+	// "private keys stay on your infrastructure" and "never touch the
+	// control plane" as blanket claims. CERTCTL_KEYGEN_MODE=server
+	// breaks both claims — the control plane mints the keys directly,
+	// in process memory, and writes them to the renewal job for
+	// delivery. Pre-fix the server printed a boot WARN and started
+	// anyway, so the blanket claim was silently false in any deploy
+	// where the operator flipped the flag without reading their logs.
+	// Mirror the Phase-2 SEC-H3 DemoModeAck pattern: refuse to boot
+	// in server-keygen mode unless the operator has explicitly
+	// acknowledged the demo posture via CERTCTL_DEMO_MODE_ACK=true.
+	// Bypass for tests that legitimately exercise the server-keygen
+	// path: those construct Config directly without going through
+	// Validate(), so this gate doesn't fire there.
+	if c.Keygen.Mode == "server" && !c.Auth.DemoModeAck {
+		return fmt.Errorf(
+			"CERTCTL_KEYGEN_MODE=server is demo-only — the control plane mints private keys in process memory, " +
+				"breaking the 'keys never touch the control plane' production posture. Set " +
+				"CERTCTL_DEMO_MODE_ACK=true + CERTCTL_DEMO_MODE_ACK_TS=$(date +%%s) to acknowledge, " +
+				"OR set CERTCTL_KEYGEN_MODE=agent (the default) for production")
+	}
 
 	// SCEP fail-loud startup gate (H-2, CWE-306).
 	//