Replace strict Conventional Commits PR-title check with a sanity check

[Pawansingh3889]

The previous pr_title.yml workflow used amannn/action-semantic-pull-request@v5 to enforce strict Conventional Commits format on every PR title (feat:, fix:, etc.) plus a lowercase-subject regex. That conflicted with the project's actual title convention ("Add Wxxx rule-name (description)", "Fix Snnn false negatives ...") and so the validate check went red on every merged PR (e.g. #46, #47, #48, #49). Branch protection does not require this check, so the merge button stayed enabled and the failures shipped silently in the merged commits' status timelines.

Replacing the strict format enforcer with a small inline shell script that catches the actual problems we have hit in practice:

1. Empty / whitespace-only titles -- noticeable in gh pr list.
2. Auto-generated branch-name titles (e.g. "Feat/w024-select-distinct -suspicious") that GitHub picks when an async PR-template render wipes out a freshly typed title. Matches a regex of the form ^[A-Z][a-z]+/[a-z0-9-] which catches the real cases without touching anything that looks like a sentence.
3. Titles longer than 100 characters, which truncate badly in the UI.

Project titles like "Add W024 select-distinct-suspicious (...)" pass the sanity check; titles like "Feat/foo-bar" fail. Verified the regex matrix locally before committing.

To restore stricter enforcement later, swap the inline script back for amannn/action-semantic-pull-request@v5 and adjust the project title convention to start with feat: / fix: prefixes.

What changed

Related issue

If this adds or changes a rule

• Rule ID is the next unused one in its family (E00N / W0NN / S00N / P00N)
• Rule class registered in sql_guard/rules/__init__.py
• Fixture line in tests/fixtures/errors.sql or warnings.sql
• "Fires on bad SQL" test in tests/test_rules.py
• "Does NOT fire on safe SQL" test (using tmp_path)
• README rule table + Key Numbers count updated
• Rule is not a removal or rename of an existing rule (breaking change — see GOVERNANCE.md)

Tests

• pytest -q passes
• ruff check . passes

Checklist

• One logical change per commit, conventional commit style (feat:, fix:, docs:, chore:, ci:, test:, style:)
• CHANGELOG.md [Unreleased] entry if user-facing

Anything else reviewers should know