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

[mhenrixon]

Summary

Backport of zoolutions/phlex-reactive#220.

Adds a Phase A0: merge conflicts phase to /github-review-pr that runs BEFORE the CI-failures phase, so CI is diagnosed against the post-merge reality instead of a stale base:

• Detection: gh pr view --json mergeable,mergeStateStatus,baseRefName; on UNKNOWN, a local fallback that tests the PR's ACTUAL head (git fetch origin pull/<PR>/head + FETCH_HEAD, ref verification via git rev-parse --verify before trusting exit codes, git merge-tree --write-tree --name-only output as the work list).
• Resolution runbook: gh pr checkout on a clean tree, MERGE never rebase (shared branch — no force-push), semantic per-file rules, verification gates before pushing, plain git push.
• Loop-back rules: a new conflict mid-pass (base moved) loops back to A0; Phase C re-checks mergeability before reporting and reports a Phase A0 summary + end-state mergeability.
• /github-review-failures gains a detection-only pre-flight: on CONFLICTING it stops and hands off to the orchestrator (its toolset deliberately excludes the merge machinery); on UNKNOWN it proceeds.
• allowed-tools frontmatter in the orchestrator now covers every command the new text instructs (gh pr checkout, git status/switch/fetch/merge/merge-tree/rev-parse, bundle install, bun, cd).

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

• lib/docs_kit/version.rb: rake release[X.Y.Z] commits the bump directly on main (the task aborts off-main), so the "a version conflict means the branch bumped on purpose" rule holds here verbatim.
• Lockfiles: the gem root's Gemfile.lock is gitignored (can never conflict); docs/Gemfile.lock IS tracked (deployable dogfood app) → take base + bundle install in docs/. docs/bun.lock is tracked too (CI runs bun install --frozen-lockfile) → take base + bun install.
• No generated-artifact rules: docs-kit tracks no minified/build outputs (docs/app/assets/builds/* and tailwind.sources.css are gitignored), so phlex-reactive's min.js/vendored-twin rules were dropped. The vendored Lucide SVGs under docs/app/assets/svg/icons/** got a take-one-side-wholesale rule instead.
• Append-only registries: docs/app/models/doc.rb's page lines and docs/config/routes.rb.
• Verification gates match CI: bundle exec rake (the rake (Ruby 3.x) job) for gem files; cd docs && bundle exec rspec (the docs site job) plus docs' own rubocop (with explicit files — the gem's rubocop deliberately excludes docs/) for docs files.
• Kept the failures command's existing check-type table, model pins (opus/sonnet), and repo-specific notes intact.

Backport of zoolutions/phlex-reactive#220.