docs: standardize on yymmdd- timestamp prefix format

Naming Convention Updates:
- Renamed history/2026-01-06-semantic-document-validation → history/260106-semantic-document-validation
- Documented yymmdd- format convention in history/README.md and roadmap/README.md
- Updated all date references in WORKPLAN.md and DONE.md
- Fixed SCHEMA_MANAGEMENT_GUIDE.md references to use yymmdd- format

Convention Details:
- Format: yymmdd-topic-name (e.g., 260106-semantic-document-validation)
- Benefits: Concise while maintaining chronological sorting
- Examples documented in both README files
- Applies to both roadmap/ and history/ directories

This establishes a consistent timestamp prefix convention that Claude and its agents should follow.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

history/260106-semantic-document-validation/DONE.md

@@ -1,6 +1,6 @@
 # Completed: Semantic Document Validation
 
-**Date Completed**: 2026-01-06
+**Date Completed**: 260106 (2026-01-06)
 **Topic**: Semantic Document Validation for x-markitect Schema Extensions
 
 ---
@@ -154,4 +154,4 @@
 ---
 
 **Topic Status**: COMPLETED AND ARCHIVED
-**Archive Location**: `history/2026-01-06-semantic-document-validation/`
+**Archive Location**: `history/260106-semantic-document-validation/`

history/260106-semantic-document-validation/WORKPLAN.md

@@ -576,7 +576,7 @@ watch -n 2 'markitect validate draft.md --schema api-documentation-schema-v1.0.m
 
 ## ✅ COMPLETION SUMMARY
 
-**Date Completed**: 2026-01-06  
+**Date Completed**: 260106 (2026-01-06)
 **Status**: All 6 phases completed successfully
 
 ### Implementation Results
@@ -660,4 +660,4 @@ watch -n 2 'markitect validate draft.md --schema api-documentation-schema-v1.0.m
 ✅ >90% test coverage for validator modules  
 ✅ Complete documentation with examples for each schema type
 
-**Topic Status**: CLOSED - Moved to history on 2026-01-06
+**Topic Status**: CLOSED - Moved to history on 260106 (2026-01-06)

history/README.md

@@ -5,6 +5,16 @@ This history directory contains old planning directories for roadmap topics.
 - See ../roadmap for current and future topics
 - 2025 has not been using using topic directories but various files
 
+## Naming Convention
+
+**Directory Format:** `yymmdd-topic-name`
+
+- Use 2-digit year prefix (e.g., `260106-` for 2026-01-06)
+- Lowercase topic names with hyphens
+- Examples: `260106-semantic-document-validation`, `260105-schema-of-schemas`
+
+This convention keeps names concise while maintaining chronological sorting.
+
 ## Purpose
 
 This historical documentation serves multiple purposes: