feat(native): label the checks that gate a pull request's merge

[omartelo]

What

The native Pulls screen's Checks tab marks the checks that gate the merge with a Required pill, the way GitHub's own checks list does.

How

• gh pr view --json statusCheckRollup does not carry isRequired (it takes the pull request as an argument), so PullRequestDetail makes one GraphQL call for it, only when the PR reports checks. It covers both rulesets and classic branch protection.
• Checks are matched by name + URL: a name repeats across workflows (test runs in both ci and native).
• A failed lookup fails the detail read; no silent fallback.
• CheckItem gains required, mirrored in native/lichclient and frontend/src/lib/api-types.ts. The web UI does not render it (maintenance mode).
• The pill reuses paletteChip; the word lands in all 7 native locales.

Notes

• Measured against chore: move the repository to the lichdotdev organization #818: both test checks (ci and native) read required, which is GitHub's answer: the main ruleset requires context test with no app pinned.
• The query reads the first 100 checks; a check beyond that never reads Required.
• pr-handoff.test.ts fixture gains required: false: the contract changed (new required field on CheckItem).

Verification

• go test ./internal/project/ (3 new tests), go vet ./internal/..., gofmt
• native/: gofmt, go vet ./..., go test ./...
• frontend: biome ci ., tsc -b, vitest run src/lib/pulls
• Not taken: a -shot of the tab (it cannot navigate to a PR's Checks tab).