MCP `branches` lists a repository's tags with `kind: "tags"`, newest first, with no new tool #5034

Intent: a harness preparing a release sees what is already tagged. Replaces #4900, which added a tags tool and was parked twice because tools/list had no room left.

Done:

• [ ] MCP branches takes kind (branches, the default, or tags); with tags it answers the tags as GET /api/v1/repos/:owner/:name/tags does, newest first, calling what that route calls, for the same caller
• [ ] kind: "tags" with name or action is refused with one sentence; an unknown kind is refused
• [ ] docs/MCP.md's branches row says it
• [ ] test/mcp-tags.test.ts: tags listed newest first; a private repository's tags are not found for a stranger; the refusals above
• [ ] test/mcp-size.test.ts passes without raising either ceiling

Files: src/http/mcp.ts (the branches tool only), docs/MCP.md, test/mcp-tags.test.ts. Risk: keep the new schema property's description to a few words; do not change the tags route.

Parked by the crew: the check failed ("a burst of errors: the board says so, one page, no more; clear for five minutes: one Recovered, all fine") and the decider saw no point in another try (pull #5095 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.