feat: migrate PR fetch to GraphQL to expose required checks (+ review decision, PR number)

[davidtaing]

Problem

PR enrichment uses gh pr list --json number,headRefName,isCrossRepository,statusCheckRollup
(ghFetchPRChecks in checks.go). That canned JSON shape can't express:

• which checks are required (branch-protection required status checks) — the
  per-check entries carry only name/status/conclusion/state/workflowName,
  no required flag.
• review decision (APPROVED / CHANGES_REQUESTED / REVIEW_REQUIRED) —
  needed by the notifications work (feat: desktop notifications for PR review + CI state changes (watch mode) #55).

What's possible

A custom gh api graphql query returns all of it in one call. Confirmed against
PR #56:

ci                 isRequired=true
build (ubuntu)     isRequired=false
build (macos)      isRequired=false
changes            isRequired=false
CodeRabbit         isRequired=false

statusCheckRollup.contexts nodes expose isRequired(pullRequestNumber: N) on
both CheckRun and StatusContext; the same query also yields reviewDecision
and number.

Proposed

Replace the gh pr list --json call in ghFetchPRChecks with a gh api graphql
query returning, per open PR: number, headRefName, isCrossRepository,
reviewDecision, and the rollup contexts with isRequired. Carry the new fields
through prStatus / Worktree:

• Check grows a Required bool.
• prStatus (and Worktree) gain Number int and ReviewDecision string.

This is foundational — it unblocks three things at once:

1. Required-check marker in --checks (this issue's headline).
2. PR number display.
3. reviewDecision for the feat: desktop notifications for PR review + CI state changes (watch mode) #55 notification tiers.

Display (the design fork — why hitl)

How to mark a required check in the --checks rows — pick one, keep it lean; the
rollup glyph stays as-is:

• bold the required rows, or
• a * suffix / req tag, or
• a --required-only filter that hides optional checks.

Design notes

• Repo resolution is the main wrinkle. gh pr list resolves the repo from
  cmd.Dir (the worktree path). gh api graphql does not substitute
  {owner}/{repo}, so we need owner/name explicitly — e.g. one cached
  gh repo view --json owner,name per repo dir, or parse the remote.
• Keep all gating/caching/cap unchanged: filter-gated, 5-project cap, 15s cache,
  hardenedGitEnv, 4s timeout. Still one subprocess per repo.
• isRequired takes the PR number, which the query has in scope per node.
• Preserve current normalisation: checkStateOf, cross-repo skip,
  empty-rollup → neutral.
• Tests mirror checks_test.go: unmarshal a sample GraphQL payload into the same
  prStatus/Check shapes; add a required-flag case.

Cut-line / slicing

Could split into: (a) an afk-able fetch migration that exposes the fields with
no display change (output stays byte-identical), then (b) the required-check
display and (c) PR number as small follow-ups.

Note

The PR number is already in today's JSON and merely discarded, so it can ship
as a tiny standalone change independent of this migration — this issue just also
surfaces it through the new path.