GET a repository's decisions pages: `limit` (1–100, default 30) and a `next_cursor`, trevato/forge's first page under 40 KB #6224

Measured 2026-10-26: GET /api/v1/repos/trevato/forge/decisions answers 106 KB. It always reads 100 rows (src/http/api.ts:3887) and has no way to read further back.

Done when:

• [ ] The route takes limit (a whole number 1–100, default 30; anything else is 422 invalid_limit) and cursor (the next_cursor of the page before; a bad one is 422 invalid_cursor), and answers { decisions, week, next_cursor }.
• [ ] Rows the caller may not see are still filtered, and a page is never short only because of that filtering.
• [ ] The OpenAPI document and MCP decisions_by, if they describe this read, say the same.
• [ ] Tests: default 30, paging to the end, both 422s.

Files: src/http/api.ts, src/core/decisions.ts, the OpenAPI source, a test.

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.