refactor(praxis): split comment hygiene into its own skill (#39)

[Jartans-Familiar]

Closes #39.

What

Splits comment hygiene out of code-hygiene into its own comment-hygiene
skill, so a diff that only needs the comment rules (KEEP/DELETE/EXEMPT/
CONDITIONAL, tombstone and retold-fact detection, comment density) doesn't
depend on the model separately deciding code-hygiene's dead-code/naming/
faking-done content is also relevant to trigger it -- the concern #39 raised.
find-duplicate-comments.js moves with it (it only serves the Retold-fact
check).

Cross-references updated so the split doesn't leave dangling pointers:
code-structure, readable-code, docs-patterns and general-reviewer
(both its frontmatter skills: list and the Important/Minor bullets that
were comment-specific).

Domain-owner clause

Artifact-only. Every sentence in comment-hygiene/SKILL.md is existing,
already-approved code-hygiene content relocated verbatim; the only new
prose is mechanical connective tissue (a one-line "split from code-hygiene"
note, and cross-file pointer edits so code-structure/readable-code/
docs-patterns/general-reviewer resolve to the new skill instead of the
old one). No new rule, test, or verdict is introduced.

Not bundled here: the issue's own comment asks to "Add no-narrativization
anti-pattern." That's new doctrine, not a relocation, so it needs a content
owner's sign-off before it lands -- routing to Ferro (engineering practice)
rather than deciding it myself. Flagged, not actioned, in this PR.

Manifest / Codex face

• praxis bumped 1.4.0 -> 1.5.0 (new skill, MINOR per RELEASING.md).
• Root .claude-plugin/marketplace.json description for praxis still
  describes the plugin as shipped ("code review with specialized reviewers,
  and project conventions") -- no update needed.
• Codex face: neither .claude-plugin/plugin.json nor .codex-plugin
  enumerates skills (Codex defaults to ./skills, matching our layout), so
  there's no manifest entry to add for the new skill directory.
  general-reviewer.toml regenerated via scripts/generate-codex.py
  because its source .md body and skills: frontmatter changed (the
  comment-hygiene skill reference and the reworded bullets).

Verification

make verify   # clean
make lint     # clean (lychee-docker/actionlint-docker skip in this sandbox --
              # no Docker socket access; codespell/shellcheck/markdownlint-cli2/
              # zizmor/pre-commit-hooks all pass)

Task JAR-413.

[Jartans-Familiar]

Path dependency worth naming before either of these merges. This renames plugins/praxis/skills/code-hygiene/ to comment-hygiene/, moving find-duplicate-comments.js. #52 adds that script's test suite at tests/plugins/praxis/skills/code-hygiene/find-duplicate-comments.test.js, which mirrors the source path and requires the source through it.

Whichever merges second needs the rebase, and it is a two-line one: the test file moves to tests/plugins/praxis/skills/comment-hygiene/ and its require path follows. No test content changes.

I would rather this one go first, since it is the rename and #52 is additive -- but either order works as long as the second one rebases rather than merging over it. Merged over, the require resolves to nothing and the whole Node half of make test fails on load.

Separately, and not a blocker here: this implements JAR-390, which is sitting in backlog assigned to me and was not dispatched. I will reconcile the issue against this PR rather than have anyone redo the work.