ci: add prettier and a pull-request format workflow

[ben-everly]

Summary

Adds Prettier to the marketplace repo and wires it into CI, then formats the
existing tree.

• Config — .prettierrc sets printWidth: 80 and proseWrap: "always", so
  markdown prose is hard-wrapped rather than left as long single lines.
  .prettierignore excludes node_modules/, package-lock.json, and all
  CHANGELOG.md files (release-it owns those).
• Scripts — npm run format / npm run format:check in a new root
  package.json, which also picks up the release-it dependencies that were
  previously implicit.
• Workflow — .github/workflows/format.yml runs on pull_request. Same-repo
  PRs get formatted and the result committed back to the branch; fork PRs get a
  read-only format:check instead, since their token can't push.
• Formatting pass — 145969a applies Prettier across the repo (37 files).

Reviewer hints:

• The bulk of the diff is mechanical reflow. The parts worth actual attention are
  .github/workflows/format.yml, package.json, .prettierrc, and
  .prettierignore.
• e4c4b15 repairs two spots where the reflow changed meaning rather than just
  shape: an indented <<'EOF' heredoc terminator in the commit skill (an
  indented delimiter doesn't terminate a non-<<- heredoc, so a verbatim copy of
  that example would have failed), and a paragraph split in the bug-report
  skill's output template.
• Verified the rest of the reflow is semantically inert: every changed file's YAML
  frontmatter parses to identical values before and after, no prose words changed,
  no ordered lists renumbered, no list nesting re-parented, and fenced-block
  contents are unchanged after de-indentation.

Known limitation, accepted for now: the auto-format commit is pushed with
secrets.GITHUB_TOKEN, and GitHub does not create workflow runs for pushes made
with that token. So when the job pushes a formatting commit, the resulting head
SHA gets no check runs. The content itself is fine (it's Prettier's own output),
but if Format is ever made a required status check, an auto-formatted PR will
hang on "Waiting for status to be reported" — and any future workflow on
pull_request won't run on that SHA either. Fixing it means pushing with a PAT
or app token. Latent today; worth revisiting before adding required checks.

Breaking changes

None. No skill, command, or agent behavior changes. Plugin content is reflowed
only — the two semantic drifts the reflow introduced are repaired in e4c4b15.

Note that merging this bumps a patch version of every plugin whose files were
reformatted (brainpower, conventional-branches, development-workflow,
git-flow, intelephense, product-discovery, ticket-branches), since each
.release-it.json scopes commits to its own directory and the angular preset
bumps patch for any commit in that path.

Architecture impact

proseWrap: "always" at 80 columns is now the binding convention for all
markdown in the repo, including every SKILL.md. Future skills should be
authored wrapped, and any generated or pasted markdown will be rewrapped on the
next PR.

Skill markdown is read by the model as raw text, so reflow is not purely
cosmetic here the way it is in most repos — anything whose exact byte layout
matters (heredocs, code meant to be emitted verbatim, templates where line
breaks carry meaning) needs a <!-- prettier-ignore --> guard. e4c4b15 is the
first instance of that class; it won't be the last.

Related issues

SIDE-200