refactor(checkers): auto-discover checker modules, drop the barrel

[pirr]

Replace the hand-curated list in src/pgsleuth/checkers/init.py with a pkgutil-based auto-discovery loop. Every submodule under pgsleuth.checkers (excluding base and any _-prefixed helper) is imported on package load, which runs each module's register(...) call and populates the global registry.

Why: every new-checker PR was appending an import to init.py, guaranteeing a textual conflict between any two open checker PRs. The barrel was also the single most common new-checker footgun (documented as such in docs/writing_checkers.md): forget the import and the rule is silently invisible. Auto-discovery removes the file that has to be edited and the bug it enabled.

Add tests/test_auto_discovery.py with two assertions:

• registry is non-empty after import (sanity).
• every non-framework module under pgsleuth.checkers has registered at least one Checker (catches the new most-common footgun: forgetting at the bottom of the module).

Update docs/writing_checkers.md: drop the 'add to barrel' step, document the auto-discovery convention and the _-prefix escape hatch for non-checker helpers.

After this, a new-checker PR touches only:

• src/pgsleuth/checkers/.py
• docs/rules/.md
• tests/checkers/test_.py
• README.md (one row in the Checks table)

The README is now the only remaining shared file. Conflicts on the Checks table can be eliminated separately by generating it from the registry.