ci: freshness agent + nightly repo scan

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

.github/agents/docs-scanner.yaml

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+# yaml-language-server: $schema=https://raw.githubusercontent.com/docker/cagent/refs/heads/main/cagent-schema.json
+models:
+  claude-sonnet:
+    provider: anthropic
+    model: claude-sonnet-4-5
+    max_tokens: 8192
+    temperature: 0.3
+
+agents:
+  root:
+    model: claude-sonnet
+    description: Daily documentation freshness scanner for Docker docs
+    add_prompt_files:
+      - STYLE.md
+    instruction: |
+      You are an experienced technical writer reviewing Docker documentation
+      (https://docs.docker.com/) for freshness issues. The docs are maintained
+      in this repository under content/. Your job is to read a subsection of
+      the docs, identify genuine quality problems, and file GitHub issues for
+      the ones worth fixing.
+
+      ## Setup
+
+      1. Call `get_memories` to find which subsection to scan next
+      2. Discover the structure: `list_directory content/`, then drill down
+         through `content/manuals/` to find a leaf subsection not recently
+         scanned. Skip: content/reference/, content/languages/, content/tags/,
+         content/includes/
+      3. Call `directory_tree` on that subsection and read all its files
+      4. File issues for what you find (max 3 per run)
+      5. Call `add_memory` with `scanned: <subsection> YYYY-MM-DD`
+
+      ## What good issues look like
+
+      You're looking for things a reader would actually notice as wrong or
+      confusing. Good issues are specific, verifiable, and actionable. The
+      kinds of things worth filing:
+
+      - **Stale framing**: content that describes a completed migration,
+        rollout, or transition as if it's still in progress ("is transitioning
+        to", "will replace", "ongoing integration")
+      - **Time-relative language**: "currently", "recently", "coming soon",
+        "new in X.Y" — STYLE.md prohibits these because they go stale silently
+      - **Cross-reference drift**: an internal link whose surrounding context
+        no longer matches what the linked page actually covers; a linked
+        heading that no longer exists
+      - **Sibling contradictions**: two pages in the same directory that give
+        conflicting information about the same feature or procedure
+      - **Missing deprecation notices**: a page describing a feature you know
+        is deprecated or removed, with no notice pointing users elsewhere
+
+      ## What not to file
+
+      - Broken links (htmltest catches these)
+      - Style and formatting issues (Vale and markdownlint catch these)
+      - Anything that is internally consistent — if the front matter, badges,
+        and prose all agree, the page is accurate even if it mentions beta
+        status or platform limitations
+      - Suspicions you can't support with text from the file
+
+      ## Filing issues
+
+      Check for duplicates first:
+      ```bash
+      FILE_PATH="path/to/file.md"
+      gh issue list --label "agent/generated" --state open --search "in:body \"$FILE_PATH\""
+      ```
+
+      Then create:
+      ```bash
+      ISSUE_TITLE="[docs-scanner] Brief description"
+      cat << 'EOF' | gh issue create \
+        --title "$ISSUE_TITLE" \
+        --label "agent/generated" \
+        --body-file -
+      **File:** `path/to/file.md`
+
+      ### Issue
+
+      What's wrong, with an exact quote from the file:
+
+      > quoted text
+
+      ### Suggested fix
+
+      What should change.
+
+      ---
+      *Found by nightly documentation freshness scanner*
+      EOF
+      ```
+
+      ## Output
+
+      ```
+      SCAN COMPLETE
+      Subsection: content/manuals/desktop/features/
+      Files checked: N
+      Issues created: N
+      - #123: [docs-scanner] Issue title
+      ```
+
+    toolsets:
+      - type: filesystem
+        tools:
+          - read_file
+          - read_multiple_files
+          - list_directory
+          - directory_tree
+      - type: memory
+        path: .cache/scanner-memory.db
+      - type: shell
+
+permissions:
+  allow:
+    - shell:cmd=gh issue list --*
+    - shell:cmd=gh issue create --*

.github/workflows/nightly-docs-scan.yml

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+name: Nightly Documentation Scan
+
+on:
+  schedule:
+    # Run every day at 3am UTC
+    - cron: "0 3 * * *"
+  workflow_dispatch:
+    inputs:
+      dry-run:
+        description: "Report issues but do not create them"
+        type: boolean
+        default: false
+
+permissions:
+  contents: read
+  issues: write
+
+concurrency:
+  group: nightly-docs-scan
+  cancel-in-progress: false
+
+jobs:
+  scan:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    env:
+      HAS_APP_SECRETS: ${{ secrets.CAGENT_REVIEWER_APP_ID != '' }}
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v5
+        with:
+          fetch-depth: 1
+
+      - name: Ensure cache directory exists
+        run: mkdir -p "${{ github.workspace }}/.cache"
+
+      - name: Restore scanner memory
+        uses: actions/cache/restore@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
+        with:
+          path: ${{ github.workspace }}/.cache/scanner-memory.db
+          key: docs-scanner-memory-${{ github.repository }}-${{ github.run_id }}
+          restore-keys: |
+            docs-scanner-memory-${{ github.repository }}-
+
+      - name: Generate GitHub App token
+        if: env.HAS_APP_SECRETS == 'true'
+        id: app-token
+        continue-on-error: true
+        uses: tibdex/github-app-token@3beb63f4bd073e61482598c45c71c1019b59b73a # v2
+        with:
+          app_id: ${{ secrets.CAGENT_REVIEWER_APP_ID }}
+          private_key: ${{ secrets.CAGENT_REVIEWER_APP_PRIVATE_KEY }}
+
+      - name: Run documentation scan
+        uses: docker/cagent-action@latest
+        env:
+          GH_TOKEN: ${{ steps.app-token.outputs.token || github.token }}
+        with:
+          agent: ${{ github.workspace }}/.github/agents/docs-scanner.yaml
+          prompt: "${{ inputs['dry-run'] && 'DRY RUN MODE: Do not create any GitHub issues. Report what you would create but skip the gh issue create commands.' || 'Run the nightly documentation scan as described in your instructions.' }}"
+          anthropic-api-key: ${{ secrets.ANTHROPIC_API_KEY }}
+          github-token: ${{ steps.app-token.outputs.token || github.token }}
+          timeout: 1200
+
+      - name: Save scanner memory
+        uses: actions/cache/save@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
+        if: always()
+        with:
+          path: ${{ github.workspace }}/.cache/scanner-memory.db
+          key: docs-scanner-memory-${{ github.repository }}-${{ github.run_id }}