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/principles/progressive-disclosure.md

@@ -69,7 +69,7 @@ curl -s "http://localhost:37777/api/search/observations?query=authentication&for
 ### Step 4: Refine with Filters (If Needed)
 
 **Techniques:**
-- Use `type`, `dateRange`, `concepts`, `files` filters
+- Use `type`, `dateStart`/`dateEnd`, `concepts`, `files` filters
 - Narrow scope BEFORE requesting more results
 - Use `offset` for pagination instead of large limits