Adds a release recipe for /wurk:release

[johnnyt]

What

Gives this repo a /wurk:release recipe. .claude/wurk.json carried
"release": null, so the skill refused here and every Hex prep was done by
hand.

Two files, both under .claude/:

• .claude/wurk.json gains a release block byte-identical to the one
  statifier_blocks carries: kind: "hex", version_file: "mix.exs",
  readme_pin: true. Deliberately no changelog key - changelog.mode
  here is fragments, there is no ## [Unreleased] heading, and pointing the
  skill at CHANGELOG.md would make its precondition read for a section that
  does not exist and its edit rename a heading that does not exist.
• .claude/wurk/release.md is the extension that supplies what the recipe
  cannot: the changelog.d/ precondition that replaces the unreleased-section
  read, the fragment-promotion step, the README pin note, the table of files a
  release commit touches, and the restatement that tag/publish are never the
  skill's.

The precedent this is modeled on

1562d8c ("Cuts the 0.2.0 release"), the most recent prep here. Two places
where this repo differs from statifier_blocks and the extension says so
explicitly, because copying the sibling would have been wrong:

• The heading form is ## [X.Y.Z] YYYY-MM-DD with no dash between version
  and date, not the skill's own ## [X.Y.Z] - YYYY-MM-DD.
• No link reference is added. CHANGELOG.md here has no link-reference
  block at all and 1562d8c added none; introducing one is a human's decision,
  not a mechanical release step.

Findings recorded rather than fixed

• No second version carrier in lib/ or docs/ - the version lives once,
  as @version in mix.exs. assets/package.json has a "version" field
  reading 0.2.0, and the extension names it explicitly as not a release
  carrier: it was created already carrying that string (61d647f), 1562d8c
  did not move it, and nothing asserts it against the project version. If it
  should track the Hex version that is a decision to record and a step to add,
  not something to infer from two strings matching today.
• The README pin is stale: it reads {:statifier_ui, "~> 0.1"} while
  mix.exs reads 0.2.0, because 1562d8c bumped the version file without
  the pin. With readme_pin: true the next release repairs it. Not touched
  here - this PR changes no README.

Verification

• ruby ~/.claude/skills/wurk:kit/scripts/lib/manifest.rb check -> valid: true.
  Positive control: temporarily setting kind to a bogus value also passed,
  so the lint does not inspect the release block. The substantive checks are
  therefore the direct comparison against the statifier_blocks block (equal)
  and reading the skill's kind: "hex" steps against this repo's files.
• Every acceptance criterion machine-checked against the committed tree: the
  block matches the sibling's minus changelog; the extension carries the
  files table and cites 1562d8c; the diff touches only the two .claude/
  files - no version, changelog, README, lib/ or mix.exs change.
• Quality gate: gate.rb reports applicable: false -
  "no changes under lib/, test/, config/, mix.exs, mix.lock, assets/,
  docs/wire-format.md". Docs-only under this repo's CLAUDE.md commit row; no
  gate ran locally, and CI replays it regardless.

What this PR is not

No version bump, no changelog promotion, no fragment deleted - all twelve
fragments in changelog.d/ are untouched. The release prep that uses this
recipe is a separate bead. mix.exs is untouched (sui-2ke owns it).

Refs: sui-6i0

Closes sui-6i0.