MCP `orgs`: the caller's organizations, and one organization with its teams and their members #5328

Split from #4797, whose two tries were green on their own tests and red on unrelated ones (now fixed by the two issues this waits for).

Done:

• [ ] A read-only tool orgs in src/http/mcp.ts, placed right after org_teams: without org, the caller's organizations (what GET /api/v1/user/orgs calls); with org, the organization, its teams and each team's members (what GET /api/v1/orgs/:org, …/teams and …/teams/:team/members call). Same functions, no new rules.
• [ ] A private organization answers not found to a non-member, exactly as the API does.
• [ ] One row in docs/MCP.md beside org_teams.
• [ ] test/mcp-orgs.test.ts: the caller's list; one org with a team and its member; a non-member gets not found. Each fails without the tool.
• [ ] Run test/mcp-orgs.test.ts, test/mcp.test.ts and test/mcp-size.test.ts before pushing. A red test elsewhere that main also has is named in the pull, not a reason to stop.

Files: src/http/mcp.ts, docs/MCP.md, test/mcp-orgs.test.ts. Risk: test/mcp-size.test.ts caps tools/list; keep the description to one short sentence and the schema to two optional strings.