Add Vale check for missing punctuation between sentences

Summary

Added a new Vale style rule (SentencePunctuation.yml) to detect missing punctuation between sentences in documentation. The rule flags cases like "This is a sentence This is another sentence." where punctuation is missing after the first sentence.

The rule uses two heuristic patterns:

1. Detects lowercase/digit/closer followed by space and common sentence starters (This, That, However, etc.)
2. Detects lowercase/digit/closer followed by space, capitalized word, and auxiliary verbs (is, are, has, etc.)

Key implementation details:

• Uses single-quoted YAML strings to avoid escaping issues
• Uses [ \t]+ instead of \s+ to match only same-line gaps (prevents false positives in code blocks)
• Removed ambiguous starters (The, A, An, In, On, At, To) to reduce false positives in YAML/code
• Set to warning level and paragraph scope

Review & Testing Checklist for Human

• Test on diverse documentation files: Run vale fern/products/docs/pages/**/*.mdx to check for unexpected false positives across the documentation corpus
• Verify intended detections work: Create a test file with missing punctuation cases and confirm they're caught (e.g., "The SDK is great It provides many features.")
• Check code block behavior: Verify the rule doesn't flag YAML comments or code samples inside <CodeBlock> components or fenced code blocks
• Evaluate trade-offs: The rule won't catch cases like "sentence The next sentence" because "The" was removed from starters to reduce false positives. Confirm this trade-off is acceptable.
• Consider severity level: The rule is set to warning. Decide if it should remain a warning or be promoted to error to block PRs with missing punctuation.

Test Plan

1. Run Vale on a few existing documentation pages: vale fern/products/docs/pages/navigation/overview.mdx
2. Create a test file with both good and bad examples:

   Good: "This is a sentence. This is another."
   Bad: "This is a sentence This is another."
   Bad: "The feature is useful It helps developers."

3. Run vale test-file.mdx and verify it catches the bad examples without flagging the good ones
4. Check that code blocks and YAML samples don't trigger false positives

Notes

• The rule uses Go's RE2 regex engine (no lookbehind support)
• Same-line limitation: Won't catch missing punctuation when sentences are split across lines
• Tested locally with Vale v3.12.0 on test files and real documentation
• CI checks passing (Vale workflow + general checks)
• Requested by: Devin Logan ([email protected]) / @devalog
• Devin session: https://app.devin.ai/sessions/3a849e21813842bf98af71119788f79a