feat(skill): ship the codeman agent skill as a Claude Code plugin from the repo's own marketplace

`.claude-plugin/marketplace.json` at the repo root makes
`/plugin marketplace add Ark0N/Codeman` work, and the one plugin it lists is
the repo itself (`source: "./"`), whose one component is `skills/codeman/`.
So `/plugin install codeman@codeman` is a third install route next to
`npx skills add` and `codeman skill install`, and the skill shows up in the
plugin directories that index Claude Code marketplaces.

Both manifests carry package.json's version: `scripts/sync-plugin-version.mjs`
rewrites them inside `version-packages`, right after `changeset version`, and
`test/plugin-manifest.test.ts` pins the equality, the skill's frontmatter name
(without it the installed skill would be named after a versioned cache dir),
and that no other plugin component (`commands/`, `agents/`, `hooks/`,
`.mcp.json`, `settings.json`) appears at the repo root, since an install would
silently ship it.

Verified with `claude plugin validate` (one expected warning: CLAUDE.md at a
plugin root is not plugin context) and a local marketplace add, install,
details, uninstall cycle against a clean checkout of this commit.

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

.claude-plugin/plugin.json

@@ -0,0 +1,22 @@
+{
+  "name": "codeman",
+  "description": "Drive Codeman, the self-hosted session manager for AI coding agents, from inside a Claude Code session: spawn worker sessions, prompt them, wait for them, read their answers, clean up. Acts only inside a Codeman-managed session.",
+  "version": "1.28.1",
+  "author": {
+    "name": "Ark0N",
+    "url": "https://github.com/Ark0N"
+  },
+  "homepage": "https://getcodeman.com",
+  "repository": "https://github.com/Ark0N/Codeman",
+  "license": "MIT",
+  "keywords": [
+    "codeman",
+    "orchestration",
+    "multi-agent",
+    "session-manager",
+    "tmux",
+    "claude-code",
+    "codex",
+    "deepseek"
+  ]
+}