list: Emit per-change forge checks via --cr-checks JSON flag

[ed-irl]

Wires the per-change checks data from each forge into 'gs log
--json' so the extension and other tooling can consume rolled-up
state and per-run detail.

• internal/handler/list: new IncludeChecks flag in the request
  bitmask; new Checks field on BranchItem; batch-loaded via
  Repository.ChecksByChange alongside change state and comment
  counts in the existing errgroup.
• log.go: new --cr-checks/spice.log.crChecks flag (negatable,
  default false) on commit log / commit log long. Honored by
  the JSON presenter; the graph presenter ignores it for now
  (UI surface lives in the extension per ext Idea: Local Merge Queue #65).
• JSON schema: change objects gain an optional 'checks' field
  with {rollup, runs[], url}, mirroring the spec at ext WIP: Revert "restack: Don't use fork point if base hash is up to date" #100.
  Rollup is one of pending/passed/failed/none; runs[] preserves
  forge-native state strings.
• testdata/script/log_checks.txt: end-to-end script covering
  failed/passed/none rollup states, per-run detail, --no-cr-checks
  omission, and the spice.log.crChecks config path.

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.}