docs: Add custom slug bug fix to CHANGELOG and implementation report

Update CHANGELOG.md with fix details in Unreleased section.
Create comprehensive implementation report documenting:
- Root cause analysis
- Code changes made
- Test results (all 13 Micropub tests pass)
- Deployment notes

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Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- **Custom Slug Extraction** - Fixed bug where mp-slug was ignored in Micropub requests
+  - Root cause: mp-slug was extracted after normalize_properties() filtered it out
+  - Solution: Extract mp-slug from raw request data before normalization
+  - Affects both form-encoded and JSON Micropub requests
+  - See docs/reports/custom-slug-bug-diagnosis.md for detailed analysis
+
 ## [1.1.0] - 2025-11-25
 
 ### Added