MCP `crew`: list a repository's crew, put an agent on it or take one off, planned first, as Settings › Crew does

Intent: the owner says who works on an app from any harness. No MCP tool calls addToCrew/removeFromCrew (src/actions/crew.ts) today.

Done:

[ ] A new tool crew takes { repo, add?, remove?, dryRun? }. Without add or remove it lists the crew's agents with their roles, read as the settings page reads them. With add or remove (an agent's username) it says what would change, and applies only when dryRun is false, through addToCrew or removeFromCrew with their refusals in words (no_such_agent, not_an_agent, unverified).
[ ] Applying needs the repository's admin with an admin token and is never offered to an agent, as archive is.
[ ] The tool is in the index (src/http/mcp/tools.ts) and docs/MCP.md through bun scripts/mcp-tools.ts. Its one-sentence description and arguments fit test/mcp-size.test.ts.
[ ] A test (e.g. test/mcp-crew.test.ts) lists, plans an add and checks nothing changed, applies it and sees the agent on the list, removes it, and checks an agent token cannot apply.

Files: src/http/mcp/tools/crew.ts (new), src/http/mcp/tools.ts, docs/MCP.md, a new test.

MCP `crew`: list a repository's crew, put an agent on it or take one off, planned first, as Settings › Crew does #7498