chore: review workflow supports merge conflicts — Phase A0 before CI failures

[mhenrixon]

Summary

Adds the merge-conflict-aware review command trio under .claude/commands/ (this repo previously had none):

• /github-review-pr — full-pass orchestrator with a new Phase A0: merge conflicts that runs BEFORE CI-failure diagnosis. Detection via gh pr view --json mergeable, with a local fallback for UNKNOWN that tests the PR's ACTUAL head (git fetch origin pull/<PR>/head + FETCH_HEAD), verifies both refs with git rev-parse --verify before trusting merge-tree exit codes, and uses git merge-tree --write-tree --name-only output as the work list. Resolution: gh pr checkout, MERGE never rebase, semantic per-file rules, verification gates before push, plain push. Loop-back rules (new conflict mid-pass → back to A0; new failure in Phase B → back to A) and Phase C report additions.
• /github-review-failures — CI-failure runbook with a detection-only merge-conflict pre-flight that STOPs on CONFLICTING and hands off to the orchestrator (this command's toolset deliberately excludes the merge machinery).
• /github-review-comments — review-thread runbook (fetch via GraphQL, categorise, fix, reply with SHAs, resolve threads).

All three carry allowed-tools frontmatter covering every command their text instructs.

Repo-specific adaptations (verified against this repo, not copied)

• Verification gates / CI check names: CI is one job type — bundle exec rake (spec + rubocop, per the Rakefile) on a Ruby matrix, so the failures table lists rake (Ruby 3.2) / rake (Ruby 3.3) / rake (Ruby 3.4); gates are bundle exec rubocop, bundle exec rspec, or bundle exec rake.
• lib/locallingo/version.rb conflict rule: kept the "the branch bumped on purpose" heuristic because it holds here — rake release[X.Y.Z] bumps, commits, and pushes directly on main (no PR).
• Lockfiles: docs/bun.lock is the ONLY tracked lockfile (root Gemfile.lock is gitignored; docs/Gemfile.lock isn't committed) — conflict rule: take base, re-run bun install --cwd docs, never hand-edit.
• No generated-artifact rules: this repo tracks no generated artifacts (docs' compiled Tailwind CSS under docs/app/assets/builds/ is gitignored), so phlex-reactive's min.js/vendored-twin rules were dropped entirely; added a note that docs/ has no PR CI safety net (its deploy fires on release).
• Ruby floor: notes that a single-matrix-version failure is a version bug, not flakiness, and fixes must not use 3.3+/3.4-only syntax (required_ruby_version >= 3.2, TargetRubyVersion: 3.2).
• API examples point at mhenrixon/locallingo; convention references cite the README / docs/AGENTS.md (no CLAUDE.md here).

Backport of zoolutions/phlex-reactive#220.