docs: verify and fix documentation against code implementation

[alexec]

Summary

This PR verifies and fixes documentation discrepancies found when comparing documentation against the actual code implementation.

Changes

• Task matching: Fixed documentation to clarify tasks are matched by filename (without .md extension), not by task_name in frontmatter
• Removed -t flag: Removed all references to non-existent -t flag for conditionally emitting task frontmatter
• Language field: Clarified that language/languages field in task frontmatter is metadata only and does NOT filter rules
• Consistent plural usage: Updated all examples to use languages: (plural) with array format [ go ] instead of singular language: go
• Selector consistency: Updated all selector references to use languages (plural) - both in frontmatter selectors and command-line flags (-s languages=go)
• Frontmatter emission: Clarified that task frontmatter is always automatically included when present
• Task name requirement: Updated to clarify task_name is optional (metadata only), tasks are matched by filename

Files Changed

• All documentation files in docs/ directory
• README.md

Testing

• All tests pass
• Linting passes with no deadcode found