shamhub: Implement ChecksByChange with full rollup + runs + URL seeding

[ed-irl]

Implements the per-change checks API for the in-process forge so
script tests and forge-suite tests can exercise the new payload
without going to a real CI system.

• shamChange grows ChecksRuns and ChecksURL fields alongside the
  existing ChecksState. Backward compatible: SetChangeChecksState
  still works for tests that only seed a rollup.
• SetChangeChecks(owner, repo, n, *forge.ChangeChecks) seeds the
  full structure; ChecksByChange(owner, repo, ns) reads it back,
  treating zero-value ChecksState as ChecksNone (no checks
  configured/reported, distinct from passing).
• New POST /{owner}/{repo}/change/checks-by-change endpoint
  mirrors the comment-counts batch shape. Returns one nil slot
  per unknown change so callers can correlate by index.
• forgeRepository.ChecksByChange dispatches to the endpoint and
  parses the response back into forge.ChangeChecks.
• parseChecksState learns 'none' so the wire round-trip matches
  the new ChecksState value added on the schema branch.
• shamhub test-script CLI gains 'shamhub set-checks <owner/repo>
  ' for seeding rich payloads from .txt scripts.

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • forge: Add ChecksByChange and ChecksNone for per-change checks output #1263
    • shamhub: Implement ChecksByChange with full rollup + runs + URL seeding #1264 ◀
      • github: Implement ChecksByChange with per-run detail and checks URL #1265
        • gitlab: Implement ChecksByChange with per-job pipeline detail #1266
          • bitbucket: Implement ChecksByChange with per-status build detail #1267
            • list: Emit per-change forge checks via --cr-checks JSON flag #1268
              • docs: Document forge checks JSON output and add changelog entry #1269

_{Change managed by git-spice.}