forge: Add ChecksByChange and ChecksNone for per-change checks output

[ed-irl]

Lays the schema foundation for emitting per-change CI/build checks
in the gs ll JSON output. The actual rollup mappings + runs[] data
for each forge land in follow-up branches on this stack.

Adds:

• forge.ChecksNone, a new ChecksState value for 'no checks
  configured/reported' (distinct from ChecksPassed). The card
  indicator hides on none; pending/failed surface visually.
• encoding.TextMarshaler/Unmarshaler on ChecksState so the state
  serializes as 'pending'|'passed'|'failed'|'none' in JSON.
• forge.ChangeChecks{Rollup, Runs, URL}: rollup drives the card,
  Runs preserves forge-native state strings for fine-grained
  surfaces (e.g. a future diff-view preamble), URL is the
  forge's per-change checks page.
• forge.CheckRun{Name, State, URL}: per-check detail. State stays
  a string (forge-native vocabulary) so the upstream API doesn't
  have to enumerate every forge's taxonomy.
• Repository.ChecksByChange(ids), a batch method mirroring
  CommentCountsByChange.

Each forge's ChecksByChange is stubbed to return a slice of nils
(equivalent to 'no data' under the contract) so the schema commit
builds standalone; real implementations land per-forge in the
follow-ups.

[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.}