test: improve frontmatter parsing test coverage

Add exhaustive test cases for frontmatter_value_byte_range and
related key helpers in skill/helpers.rs.

Scenarios covered:
- Unquoted values with/without comments
- Double and single quoted values
- Quoted values with comments
- Values containing colons
- CRLF line endings
- Indented keys
- Malformed YAML (unclosed quotes)
- Multiline/nested values (expected None)

Updated CHANGELOG.md.

Verified logic using a standalone Rust script as cargo test
was restricted by environment dependency issues.

Author: avifenesh

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Improved frontmatter parsing test coverage**: Added exhaustive unit tests for `frontmatter_value_byte_range`, `frontmatter_key_offset`, and `frontmatter_key_line_byte_range` in `agnix-core`. Covers unquoted/quoted values, comments, CRLF endings, indented keys, and malformed input.
 - **`build_lenient()` on `LintConfigBuilder`**: New builder terminal that runs security-critical glob pattern validation (syntax, path traversal, absolute paths) while skipping semantic warnings such as unknown tool names and deprecated field warnings. Intended for embedders that accept future or unknown tool names without rebuilding. `ConfigError::AbsolutePathPattern` variant added for absolute-path glob patterns (#475)
 - **Expanded autofix coverage**: Added `with_fix()` autofix support to 38 additional validation rules across AGM, AMP, AS, CC-AG, CC-HK, CC-PL, CC-SK, CDX, COP, CUR, GM, KIRO, MCP, OC, PE, and REF categories, bringing total fixable rules from 59 to 97 (42% of all rules)
 - **Kiro steering file validation**: 4 new validation rules (KIRO-001 through KIRO-004) for `.kiro/steering/*.md` files - validates inclusion modes (`always`, `fileMatch`, `manual`, `auto`), required companion fields, glob pattern syntax, and empty file detection