The repository map's API names what holds each ready issue: hot_file with the file, run_cap, open_pull and the rest #6441

Flow work for #5930. The planner is told "0 can start now" while the map lists 14 ready issues, and nothing says why. src/crew/held.ts computes it (heldBy), but GET /api/v1/repos/:owner/:name/map does not return it, so a planner keeps planning into files that are already hot.

Done when:

• [ ] Each entry in the map's ready list carries held: null or { by: <HoldName>, detail }. For hot_file, detail names the file and its count of open pulls. The map also carries startable: the numbers no filter holds.
• [ ] It reuses heldBy and hotFiles and adds no git call on the request. The route stays under its current timing test.
• [ ] A reader who may not see a private repository's issues sees no detail naming them.
• [ ] A test seeds one issue held by a hot file and one free, and checks held and startable.

Files: the map route in src/http/api.ts (around /api/v1/repos/:owner/:name/map), src/crew/held.ts (an export only if needed), a new test/map-held.test.ts.