fix(plugin): move the Claude Code plugin into plugins/codeman so an install no longer runs npm install

With the repo root as the plugin root, `claude plugin install codeman@codeman`
copied the whole checkout into its cache and, because that root carries a
package.json, ran an npm install there: 832 MB, 511 packages and this repo's
postinstall build on every installer's machine (measured from a clean worktree
of the previous commit). A plugin root must be a directory without one.

The plugin is now `plugins/codeman/`: its manifest, a README, and a MIRROR of
`skills/codeman/`. A mirror rather than a symlink because the install copies
the plugin directory and a link pointing outside it would dangle; a mirror
rather than the source because every install path, injector and doc already
names `skills/codeman/`. `scripts/sync-plugin.mjs` (replacing
sync-plugin-version.mjs) mirrors the skill and syncs both manifest versions
inside `version-packages`; `test/plugin-manifest.test.ts` pins byte-identity,
the versions, the absence of a package.json in the plugin root and that the
repo root `.claude-plugin/` holds only the marketplace manifest.

`claude plugin validate --strict` now passes for both the plugin and the repo
root. Install commands are unchanged.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

package.json

@@ -36,7 +36,7 @@
     "check:public-assets": "node scripts/check-public-assets.mjs",
     "capture:subagents": "node scripts/capture-subagent-screenshots.mjs",
     "changeset": "changeset",
-    "version-packages": "changeset version && node scripts/sync-plugin-version.mjs && npm install --package-lock-only && node scripts/check-lockfile-sync.mjs",
+    "version-packages": "changeset version && node scripts/sync-plugin.mjs && npm install --package-lock-only && node scripts/check-lockfile-sync.mjs",
     "check:lockfile": "node scripts/check-lockfile-sync.mjs",
     "knip": "npx --yes knip@latest --config config/knip.json",
     "release": "changeset publish",