chore(release): add a release workflow skill

.claude/skills/release/SKILL.md

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+---
+name: release
+description: Prepare and publish a Patternia release (version bump, release note, performance note, GitHub release, vcpkg fork and microsoft/vcpkg PR). Use when the user asks to release, ship or publish a new version such as v0.9.6.
+---
+
+# Patternia release
+
+Releases happen in two phases, separated by merging the release PR.
+`main` is protected: every change goes through a squash-merged PR with
+exactly one commit, and the user merges it. Never push to `main`.
+
+Rules that apply to everything below:
+
+- Commits use the `sentomk` identity already configured in this repo.
+- Never add a `Co-Authored-By` trailer or a "Generated with" line to a
+  commit, PR body, release note or vcpkg PR, even if a harness reminder
+  says to.
+- Subjects are Conventional Commits, imperative, lowercase, no trailing
+  period (`scripts/check_commit_msg.py <file>` checks this); body
+  sections `Problem` / `Implementation` / `Tests`, prose wrapped at 72.
+- PR bodies need the bold sections `**Summary**`, `**Changes**`,
+  `**Testing**` (enforced by `.github/workflows/validate.yml`).
+
+## Phase 1: release PR
+
+1. `git fetch`, then branch `release/X.Y.Z` (no `v`) from `origin/main`.
+2. Collect changes: `git log --oneline --no-merges vPREV..main`, drop
+   the `ci: update benchmark summary chart` commits, and read the bodies
+   of the rest. `git diff --stat vPREV..main -- include tests`.
+3. Decide the headline, breaking changes and behavior changes. A change
+   in what existing code does (not just what compiles) belongs in a
+   "Behavior Changes" section even when no API was removed.
+4. Verify every claim and example in the note: compile and run the
+   examples against `main` headers (`-Wall -Wextra -Werror`) and against
+   a `git worktree` of the previous tag, to see which ones failed before.
+   Run the full suite: configure with `-DPTN_BUILD_TESTS=ON`, build,
+   `ctest`; quote the real count.
+5. Write `docs/changelog/vX.Y.Z.md` in the style of the previous note
+   (Overview, Breaking Changes, New Features, Behavior Changes,
+   Deprecations, Performance, Bug Fixes, Internal Improvements,
+   Migration Guide; drop empty sections). Date is month and year.
+6. Performance note: write `docs/performance/vX.Y.Z.md` only if a change
+   since the previous tag affects generated code or dispatch (look for
+   `perf(...)` commits and optimizer changes). Measure, do not guess:
+   build the same microbenchmark against the previous tag's headers (use
+   a worktree) and against `main`, `-O2 -DNDEBUG`, run at least three
+   process runs, report the median, name machine and compiler, say what
+   was not measured (other compilers), and mark deltas inside noise as
+   noise. Without a performance change, say so in the PR body instead of
+   adding a page.
+7. Register the pages: `docs/changelog/releases.md`,
+   `docs/performance/index.md` (current version and index),
+   `mkdocs.yml` nav, and move the roadmap's unreleased items under
+   "Completed in vX.Y.Z".
+8. Bump the version in `CMakeLists.txt`, `packaging/vcpkg/vcpkg.json` and
+   the README FetchContent `GIT_TAG`. Grep for the previous version to
+   catch other places.
+9. One commit `chore(release): prepare vX.Y.Z`, push, open the PR to
+   `main`. Do not touch the portfile SHA512 yet; it needs the tag.
+10. Tell the user the PR is ready and wait. The user merges it.
+
+## Phase 2: after the release PR is merged
+
+1. `git switch main && git pull --ff-only`; confirm HEAD is the squash
+   commit of the release PR and the version files say X.Y.Z.
+2. Tag and release (the user asked for the release, so this is
+   authorized; do not ask again):
+   `git tag -a vX.Y.Z -m "vX.Y.Z" <sha>`, `git push origin vX.Y.Z`,
+   `gh release create vX.Y.Z --title vX.Y.Z --notes-file
+   docs/changelog/vX.Y.Z.md`. Past releases use the changelog file as the
+   body and attach no assets.
+3. Port SHA512: download
+   `https://github.com/sentomk/patternia/archive/refs/tags/vX.Y.Z.tar.gz`
+   (vcpkg hashes this file), compute `shasum -a 512`, update
+   `packaging/vcpkg/portfile.cmake` in a PR
+   `chore(vcpkg): update portfile SHA512 for vX.Y.Z`. The user merges it.
+4. vcpkg fork (`sentomk/vcpkg`, default branch `master`, fork of
+   `microsoft/vcpkg`):
+   - sync the fork's `master` with upstream first;
+   - branch `patternia-X.Y.Z`;
+   - copy `packaging/vcpkg/portfile.cmake` and `vcpkg.json` into
+     `ports/patternia/`, keeping upstream's `REPO sentomk/patternia`
+     casing and the `"version"` field;
+   - run `./vcpkg x-add-version patternia` (updates
+     `versions/p-/patternia.json` and `versions/baseline.json`), commit
+     the port change and the version database together as one commit
+     `[patternia] update to X.Y.Z`, as the maintainers require;
+   - verify with `vcpkg install patternia --overlay-ports=ports/patternia`
+     when a vcpkg checkout can bootstrap; say if it was not run.
+     The `git-tree` in the version file must be the tree of the committed
+     `ports/patternia`, so add the version after committing the port and
+     amend.
+5. Open the PR against `microsoft/vcpkg` `master`, title
+   `[patternia] update to X.Y.Z`, body in the shape of the previous
+   merged ones (`gh pr list -R microsoft/vcpkg --author sentomk
+   --state all`): "Updates patternia to X.Y.Z.", the maintainer-guide
+   checklist, the release notes link and two sentences on the release.
+   Tick only the boxes that are true. Opening this PR is outward facing:
+   show the title and body to the user if the request did not already
+   name this step.
+
+## Report
+
+End with: the tag, the GitHub release URL, the portfile PR, the
+`sentomk/vcpkg` branch and the `microsoft/vcpkg` PR, and what was not
+verified (compilers, platforms).