feat(standards): MR-based tag strategy + repository inventory (registry://repos)

Versioning standard now documents the MR flow: bump pyproject.toml/VERSION and promote CHANGELOG in the MR (referencing the version/tag, not a commit id), merge/squash, then tag the resulting commit. Adds the agent-assisted tagging procedure (wait for the go-ahead, find the version-bump commit, tag only it, ask the user to push) to both the reference page and the releasing wiki runbook. Adds the repo inventory (src/repos.json -> mcp/repos.json), the registry://repos resource + list_repos tool, the 15_repository_inventory lib page, and two MCP prompts: compliance (remediate repos) and release (agent tagging).

standards/mcp/index.json

@@ -1,6 +1,6 @@
 {
   "project": "standards",
   "type": "standards",
-  "modules_count": 15,
+  "modules_count": 16,
   "source": "hand-authored"
 }