ci(security): add dependency governance and ownership controls (Task 08)

- .github/dependabot.yml: weekly grouped github-actions updates (keeps the
  Task 05 SHA pins current). Scoped to actions only — the primary lockfile
  is bun.lock and Dependabot's npm ecosystem would desync it; npm
  dependency risk is covered by dependency-review below.
- .github/workflows/dependency-review.yml: runs on PRs with contents: read,
  SHA-pinned actions, fail-on-severity: high to block newly introduced
  high-severity vulnerabilities (exceptions documented inline).
- .github/CODEOWNERS: default owner @darrenhinde with explicit ownership of
  security-sensitive paths (workflows, dependabot, CODEOWNERS, SECURITY.md,
  scripts/validation).

Verified: dependabot.yml and dependency-review.yml are valid YAML;
dependency-review actions are SHA-pinned with contents: read; CODEOWNERS
owner is the repository owner.

.github/CODEOWNERS

@@ -0,0 +1,19 @@
+# Code owners for OpenAgentsControl
+#
+# Owners are automatically requested for review on PRs that touch matching
+# paths. Security-sensitive paths are listed explicitly so they always require
+# a maintainer review.
+
+# Default owner for everything in the repo.
+*                                   @darrenhinde
+
+# CI/CD workflows — untrusted-input execution surface.
+/.github/workflows/                 @darrenhinde
+/.github/dependabot.yml             @darrenhinde
+/.github/CODEOWNERS                 @darrenhinde
+
+# Security policy and reporting.
+/SECURITY.md                        @darrenhinde
+
+# PR validation and detection scripts consumed by CI.
+/scripts/validation/                @darrenhinde

.github/dependabot.yml

@@ -0,0 +1,19 @@
+version: 2
+
+updates:
+  # GitHub Actions — keeps workflow action versions (and their SHA pins) current.
+  # Scoped to actions only: the repo's primary lockfile is bun.lock, and
+  # Dependabot's npm ecosystem would update package-lock.json without syncing
+  # bun.lock, causing lockfile drift. Dependency (npm) vulnerabilities are
+  # instead caught at PR time by dependency-review.yml.
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 5
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+    commit-message:
+      prefix: "ci"

.github/workflows/dependency-review.yml

@@ -0,0 +1,26 @@
+name: Dependency Review
+
+on:
+  pull_request:
+    branches: [main, dev]
+
+permissions:
+  contents: read
+
+jobs:
+  dependency-review:
+    name: Dependency Review
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+        with:
+          persist-credentials: false
+
+      - name: Review dependencies
+        uses: actions/dependency-review-action@2031cfc080254a8a887f58cffee85186f0e49e48 # v4
+        with:
+          # Block PRs that introduce newly known high-severity (or worse)
+          # vulnerabilities. Document any exception inline in the PR that adds
+          # an allow-ghsas / allow-dependencies-licenses entry here.
+          fail-on-severity: high