feat: review pull requests by number

[esmuellert]

Summary

Add first-class pull request review by number without checking out a branch or changing the working tree.

:CodeDiff pr 512
:CodeDiff pr 512 --remote upstream
:CodeDiff pr 512 --base release/3.x
:CodeDiff pr 512 -- lua/codediff

Changes

• discover and fetch provider pull request refs directly through Git
  • GitHub and Azure DevOps: refs/pull/<number>/{merge,head}
  • GitLab: refs/merge-requests/<number>/{merge,head}
• prefer the provider synthetic merge commit so its parents identify the review base and head
• fall back to the PR head plus the remote default branch, with --base for non-default targets
• support --remote, the existing --repo/-C override, and Git pathspecs
• keep fetched commits under refs/codediff/pull-requests/ and force-update them after PR force-pushes
• add explicit cache cleanup commands:
  • :CodeDiff pr clean <number>
  • :CodeDiff pr clean --all
  • both support repository selection; cleanup can be scoped with --remote
• separate atomic Git operations from provider-specific pull request orchestration
• document the command, ref lifecycle, supported providers, and cleanup behavior

Benefits

• review a PR from its number with the existing merge-base Explorer
• no checkout, local branch, worktree mutation, provider CLI, or API token required
• works with existing Git remote authentication, including private repositories
• supports fork workflows by selecting the target repository remote
• cached refs remain safe for lazy/asynchronous file loading while remaining explicitly removable

Testing

• CODEDIFF_TEST_JOBS=1 ./tests/run_tests.sh — 108 spec files passed
• pull request integration suite — 12 tests passed against local bare remotes
• command E2E suite — 24 tests passed
• verified GitHub/Azure DevOps and GitLab ref layouts, merge and head fallback paths, explicit base selection, non-origin remotes, force-push updates, pathspecs, invalid/missing PRs, and branch/worktree invariants
• verified targeted cleanup does not remove another PR, remote, or unrelated refs/codediff/* namespace
• mutation-checked provider selection, force-update behavior, merge-base dispatch, targeted cleanup, and all-ref cleanup
• manually opened GitHub PR fix(compact): align zero-width hunk context #512 end-to-end without checkout