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>

roadmap/README.md

@@ -6,6 +6,16 @@ This roadmap directory contains planning directories for roadmap topics.
 - Keep current state of what's next to implement in TODO.md
 - See ../history directory for closed topics
 
+## 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-evolution`
+
+This convention keeps names concise while maintaining chronological sorting.
+
 ## Purpose
 
 This planning documentation serves multiple purposes: