feat: sync 12 new skills into catalog (total 31 skills), update manifest and README

skills/drawio-skill/references/pr-bot.md

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+# PR Diagram Bot — reviewing `.drawio` changes as pictures
+
+Goal: in CI, for every `.drawio` file a pull request touches, render the base
+version, the head version, and a colour-coded diff, then post them as a
+sticky PR comment (and the job summary) so reviewers see pictures instead of
+raw XML diffs.
+
+Read this when you're setting up (or troubleshooting) automated PR diagram
+review for a repo.
+
+## Pieces
+
+- `scripts/prdiff.py` — the script. `changed_drawios()` finds what changed
+  (`git diff --name-status`); for each file it exports base/head PNGs via the
+  draw.io CLI and, for modified files, chains `drawiodiff.py` ->
+  `autolayout.py` -> CLI export into a third diff PNG; `render_markdown()`
+  turns all of that into one Markdown report.
+- `.github/actions/drawio-diff/action.yml` — a composite action that checks
+  out full history, installs draw.io + Graphviz, runs `prdiff.py`, uploads
+  the PNGs + report as a build artifact, writes the report to
+  `$GITHUB_STEP_SUMMARY`, and posts/updates a sticky PR comment.
+- `.github/workflows/drawio-pr-diff.example.yml` — a template workflow that
+  wires the action to `pull_request` events. It ships as `.example.yml` and
+  gated with `if: false` so it does **nothing** until you copy and adapt it
+  in your own repo (see the comment at the top of that file).
+
+## Adopting it in your own repo
+
+1. Copy `.github/actions/drawio-diff/` and `skills/drawio-skill/` (or at
+   least `scripts/prdiff.py`, `scripts/drawiodiff.py`, `scripts/autolayout.py`)
+   into your repo.
+2. Copy `.github/workflows/drawio-pr-diff.example.yml` to
+   `.github/workflows/drawio-pr-diff.yml`, drop the `if: false` guard, and
+   uncomment the `pull_request: paths: ["**/*.drawio"]` trigger.
+3. Give the job `permissions: pull-requests: write` (already set in the
+   example) — the sticky comment step needs it; `contents: read` covers the
+   checkout.
+4. Push a PR that touches a `.drawio` file and watch it run.
+
+## Runner tooling
+
+The draw.io desktop CLI is Electron-based, so headless CI needs the same
+setup documented in `docs/CI.md` "Option A" of this repo: Graphviz (`dot`,
+for `autolayout.py`) and a virtual display (`xvfb-run`). The composite action
+installs both by default (latest `drawio-desktop` `.deb` + `apt-get
+graphviz xvfb`); pass `skip-tool-install: true` if your runner/container
+already provides `drawio` and `dot` on PATH.
+
+If the draw.io CLI is missing (or fails), `prdiff.py` does **not** hard-fail
+the run — the Markdown report still lists every changed file and its status,
+just without images, with a note that image export was skipped. `git`
+missing (or `--repo` not being a git repository) IS fatal, since without git
+there is nothing to diff.
+
+## How the sticky comment works
+
+The action tags its comment body with an HTML marker
+(`<!-- drawio-pr-diff-bot -->`) and, before posting, searches the PR's
+existing comments (via `gh api .../issues/<n>/comments`) for one starting
+with that marker. If found, it `PATCH`es that comment in place; otherwise it
+creates a new one with `gh pr comment`. This keeps one running comment per PR
+instead of a new comment on every push.
+
+## Running it locally
+
+```
+python3 skills/drawio-skill/scripts/prdiff.py --base origin/main --head HEAD \
+  --out-dir drawio-pr -o drawio-pr/report.md
+```
+
+`--base`/`--head` are any git refs or SHAs (`--head` defaults to `HEAD`);
+`--repo` points at a different working tree (default: current directory).
+Open `drawio-pr/report.md` to preview exactly what the PR comment will say.