docs: Update all search API documentation for simplified parameters

Update all documentation to reflect the new simplified URL parameter format:
- Replace dateRange[start]/dateRange[end] with dateStart/dateEnd
- Clarify that concepts, files, and obs_type accept comma-separated values
- Update all code examples in skill documentation
- Update comments in search-server.ts

Files updated:
- SKILL.md - Main skill documentation
- operations/*.md - 8 operation guides (observations, prompts, sessions,
  by-file, by-type, by-concept, common-workflows, help)
- principles/progressive-disclosure.md - Design pattern doc
- src/servers/search-server.ts - Code comment

All examples now use clean URLs without bracket encoding:
- Old: ?dateRange[start]=2025-11-01&concepts[]=decision
- New: ?dateStart=2025-11-01&concepts=decision

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

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

plugin/skills/mem-search/operations/help.md

@@ -106,9 +106,9 @@ Many endpoints share these parameters:
 - **limit**: Number of results to return
 - **offset**: Number of results to skip (for pagination)
 - **project**: Filter by project name
-- **dateRange**: Filter by date range
-  - `dateRange[start]`: Start date (ISO string or epoch)
-  - `dateRange[end]`: End date (ISO string or epoch)
+- **dateStart/dateEnd**: Filter by date range
+  - `dateStart`: Start date (YYYY-MM-DD or epoch timestamp)
+  - `dateEnd`: End date (YYYY-MM-DD or epoch timestamp)
 
 ## Error Handling