Add CodeRifts API governance plugin (MCP server + skill)

[zsobpeter-code]

Adds CodeRifts as a third-party plugin (local source under external_plugins/coderifts).

• MCP server: remote Streamable HTTP at https://app.coderifts.com/mcp (type: http). Also listed in the official MCP registry as io.github.coderifts/api-governance.
• Skill (api-governance): guides the agent to run a CodeRifts preflight before merging or shipping API or MCP/tool-schema changes, returning an ALLOW/WARN/REQUIRE_APPROVAL/BLOCK decision.
• Auth: CODERIFTS_API_KEY env var; discovery (initialize, tools/list) works without a key, tool calls require one. No secrets in the repo.

Validated locally: scripts/generate-plugin-index.py and scripts/validate-catalog.py both pass (Catalog OK).