docs: Document forge checks JSON output and add changelog entry

[ed-irl]

• doc/src/cli/json.md: documents the new optional 'checks' field
  on each change object in 'gs log --json'. Schema mirrors the
  implementation (4-state rollup, optional runs[] with
  forge-native state strings, optional URL), with an unreleased
  badge so the page renders the right version label until cut.
  • changelog (Added): user-facing line covering the new
    --cr-checks flag and spice.log.crChecks config.

Tracks irl-llc/git-spice-code-extension#100.

[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.}