GitHub: pull request reads and writes #14

Outcome

The factory checks, readies and comments on pull requests with the same gh calls softfac makes.

Acceptance criteria

• tracker/github.ts gains: view a pull request (repo, head repo and ref, base, draft, state, merged, head sha), mark it ready and back to draft, comment, read a branch tip sha, compare two shas, and list check runs for a sha. Each call uses the same argv as softfac.
• softfac's src/github.rs cases for these calls are ported and pass.

Depends on

• GitHub: issue reads and writes through the gh CLI #12

Notes

Reference

• softfac, the Rust factory this ports, pinned at rawphp/softfac@bf273a1. Read it from a separate clone: git clone --filter=blob:none https://github.com/rawphp/softfac /tmp/softfac-ref && git -C /tmp/softfac-ref checkout bf273a1d93651992b4fc95b4dd6c111f8f3322cf. Its docs/lifecycle.md, docs/reference/ledger.md and docs/issues.md describe the behaviour; its tests pin it.
• Design: factory/docs/design.md in this repo.
• Upstream's AGENTS.md applies. Run only the tests and typechecks for the paths you touch (vp test run <path>, vp run --filter t3 typecheck), never the repo-wide checks.
• Edit no upstream file that is not listed in factory/SEAMS. A new seam needs a line in factory/SEAMS and a reason in the PR body.
• Port behaviour, not code: port the named test cases first (TypeScript, vitest / @effect/vitest), then make them pass. Exact comment and ledger wording is ported as is.