API /map and MCP `map` name what a builder can start now: `startable`, unassigned and unheld with nothing open in front #6177

On 2026-10-26, the ready list from GET /api/v1/repos/trevato/forge/map held 126 issues: 106 waiting, 7 held and 1 working. Only 12 were startable, and counts.ready said 12. An agent reading ready overcounts by ten times.

Done when:

• [ ] The answer gains startable: the numbers of the open issues that have no assignee, are not held, parked or deferred, and have nothing open in front. It is ordered as ready is.
• [ ] counts.ready equals startable.length, and a test asserts the two agree on a fixture with waiting, held and working issues.
• [ ] ready itself is unchanged, and docs/MCP.md says how the two lists differ.

Files: src/web/pages/map.ts (mapJson), src/http/mcp/tools/map.ts, docs/MCP.md, test/map*.test.ts. Risk: keep ready as it is, because the page's Ready now section reads it.

Parked by the steward, 2026-10-26 (owner: "get us unclogged now then resume everything else"). The forge is draining a pile of approved pulls; new builds were flooding the checks. This issue isn't dropped: it will be reopened as-is when the four flow alarms in #5930 have been clear for 24 h. The list is kept in the steward's parked-2026-10-26 file.