Adopt validate-plugins action suite; pin all external SHAs (#1762)

* Adopt validate-plugins action suite; pin all external SHAs

Replaces the hand-rolled marketplace validator and bot-based bump
workflow with the shared composite actions (pinned at f846a0b).

marketplace.json:
- 62 external entries that were missing a `sha` are now pinned to
  their current upstream HEAD (resolved via git ls-remote).

Workflows:
- validate-plugins.yml: invariants I1-I11 + claude plugin validate +
  diff-gated clone-at-SHA validation of changed external entries.
  SHA-pin (I5) is a hard error. I8/I11 stay warnings until the 15
  known data issues (vendored dirs without manifests; one dotted
  name) are cleaned up.
- bump-plugin-shas.yml: bot-free weekly refresh. Validates each new
  SHA with claude plugin validate before opening one PR; works with
  the default GITHUB_TOKEN (contents:write + pull-requests:write).
- scan-plugins.yml: Claude policy scan of changed external entries.
  Non-blocking; graceful no-op if ANTHROPIC_API_KEY isn't set.

Removed:
- validate-marketplace.yml + the two TS helper scripts (superseded
  by step 11/20 of validate-plugins).

validate-frontmatter.yml is kept — it's complementary (targeted
checks on agent/skill/command files for in-repo plugins).

* Remove 5 external entries that fail validation at HEAD

Step 30 (clone at pinned SHA + claude plugin validate) fails for
these at their current HEAD:

  aiven                   Unrecognized key "logo" in plugin.json
  atlassian-forge-skills  skill YAML frontmatter parse error
  sagemaker-ai            skill YAML frontmatter parse error
  speakai                 no plugin manifest at repo root
  stagehand               no plugin manifest at repo root

These can be re-added once the upstream repos are fixed.

* Wire scan-plugins to the detailed policy prompt

Adds .github/policy/prompt.md and schema.json (the full security
review rubric — malicious code, privacy, deception, safety
circumvention, exfiltration; plus network-call and software-install
flags) and points scan-plugins at it via the policy-prompt input.

With ANTHROPIC_API_KEY now configured on the repo, scan-plugins runs
the actual policy review on changed external entries instead of
no-op'ing.

* Bump scan-plugins action pin to include L11/L12 fixes

.github/workflows/validate-plugins.yml

@@ -0,0 +1,34 @@
+name: Validate Plugins
+
+on:
+  pull_request:
+    paths:
+      - '.claude-plugin/**'
+      - '*/.claude-plugin/**'
+      - '*/agents/**'
+      - '*/skills/**'
+      - '*/commands/**'
+  push:
+    branches: [main]
+    paths:
+      - '.claude-plugin/**'
+
+permissions:
+  contents: read
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - uses: anthropics/claude-plugins-community/.github/actions/validate-plugins@f846a0bcb0e721b1f93d60e8b73e91dafc4a1e87
+        with:
+          marketplace-path: .claude-plugin/marketplace.json
+          # Official curated marketplace: SHA-pin (I5) is a HARD error.
+          # I8/I11 are warnings until the 15 known vendored-path/name issues
+          # are cleaned up (see PR body); tighten to "I1 I3" after.
+          warn-invariants: "I1 I3 I8 I11"
+          claude-cli-version: latest