MCP `tags`: a repository's tags, newest first #4900

Intent: a harness preparing a release sees what is already tagged.

Done:

• [ ] src/http/mcp.ts has a tags tool, placed right after branches, that calls what GET /api/v1/repos/:owner/:name/tags call, for the same caller
• [ ] docs/MCP.md has its row beside branches
• [ ] test/mcp-tags.test.ts: tags are listed newest first; a private repository's tags are not found for a stranger

Risks: test/mcp-size.test.ts holds tools/list under a 37,000-byte headroom, so keep the description to one short sentence. Other MCP tools are being added in parallel, so put this one exactly where named. Do not change the API routes: the tool calls the functions they call, with the same permissions.

Parked by the crew: the check stayed red through 2 automatic tries (pull #4966 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the preview's deploy failed and the decider saw no point in another try (pull #5026 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries (pull #4966 closed)
2. the preview's deploy failed and the decider saw no point in another try (pull #5026 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Replaced by #5034. Both tries failed for one reason, not the tags tool: on main, MCP tools/list is 36,963 bytes against test/mcp-size.test.ts's 37,000-byte headroom, so any new tool turns the check red. #5034 lists tags through the existing branches tool (kind: "tags") and waits for #5031, which makes room by dropping default-only annotations.