Use “convention” instead of “rule” throughout documentation

The documentation alternates between “rule” and “convention” for the same Konsistent concept, while predicates are the actual individual checks inside a convention. This makes the policy hierarchy harder to understand and should use “convention” consistently.

Relevant files:

• packages/konsistent/README.md
• docs/README.md
• docs/reference/configuration.md
• docs/reference/conditional-rules.md
• docs/guides/getting-started.md

Replace “rule” with “convention” wherever the full scoped policy is meant, including headings, filenames, links, examples, and internal identifiers that encode the documentation term. Keep “predicate” for an individual assertion.

Backward compatibility: documentation and internal terminology can be renamed or removed without compatibility shims. No konsistent.json schema change is expected; if one becomes necessary, retain the old name and behavior as deprecated and emit a non-failing CLI warning when it is used.

Start by reviewing packages/konsistent/README.md, docs/README.md, docs/reference/configuration.md, docs/reference/conditional-rules.md, and docs/guides/getting-started.md, then search those files for the scoped use of “rule.” Done means the documentation consistently uses “convention” for the full policy, preserves “predicate” for individual assertions, and updates affected headings, links, examples, filenames, and internal identifiers without changing the konsistent.json schema.