refactor(git-hooks): consolidate hook path and remove ci/ validators

[ruby-automation]

Summary

• Deletes ci/conventional_commits.sh and ci/validate_conventional_commit.sh — the validation logic now lives entirely in git-hooks/validate-commit-range and git-hooks/validate-conventional-subject (landed in EXT-29 / ci: harden Conventional Commit validators and Git test isolation #218)
• Updates .github/workflows/ci.yaml to run ./git-hooks/validate-commit-range instead of the deleted script
• Updates app/usage-rules.md to reference the new script path

The hook installer (mix git_hooks), the hook wrappers (git-hooks/commit-msg and git-hooks/pre-push), the shared validators, and all other documentation were already updated by EXT-29.

Closes EXT-30.

Test plan

• mix test (root, 35 tests) — all pass
• mix ci (full app quality gate, 405 tests) — all pass
• mix format --check-formatted (root) — clean
• No remaining active references to ci/conventional_commits.sh, ci/validate_conventional_commit.sh, or githooks/ outside historical plan/changelog files

🤖 Generated with Claude Code

[ruby-automation]

Rework complete

This addresses the blocking architecture correction raised in review.

What changed:

• ci/validate_conventional_subject.sh — canonical subject validator (was git-hooks/validate-conventional-subject)
• ci/validate_pull_request_title.sh — canonical PR title guard (was git-hooks/validate-pull-request-title)
• ci/validate_commit_range.sh — canonical commit-range checker (was git-hooks/validate-commit-range)
• ci/validate_push_refs.sh — new canonical pre-push check; implements Git's pre-push stdin protocol (<local-ref> <local-sha> <remote-ref> <remote-sha> per line), handles deletions (skip), new branches (merge-base with origin/main/main), and existing branches (remote SHA as base); preserves the GitHub Update-branch merge exception

git-hooks/ now contains only the two lifecycle adapters:

• git-hooks/commit-msg — exec "$repo_top/ci/validate_conventional_subject.sh" "$1"
• git-hooks/pre-push — exec "$repo_top/ci/validate_push_refs.sh" "$@"

mix precommit and .github/workflows/ci.yaml call ci/ scripts directly.

Tests updated:

• @subject_guard, @title_guard, @range_guard now point to ci/*.sh
• Added @push_refs_guard with four new tests (empty stdin, deletion skip, valid range, invalid range, new-branch mode)
• setup_hooks_worktree! → setup_ci_worktree! — creates ci/ in the test worktree
• Added structural assertion: git-hooks/ contains exactly ["commit-msg", "pre-push"]
• precommit_test.exs expects ./ci/validate_pull_request_title.sh and ./ci/validate_commit_range.sh

All 27 tests pass; formatting clean.