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/SKILL.md

@@ -98,9 +98,9 @@ curl "http://localhost:37777/api/prompt/5421"
 **Filters (optional):**
 - `type` - Filter to "observations", "sessions", or "prompts"
 - `project` - Filter by project name
-- `dateRange[start]` - Start date (YYYY-MM-DD)
-- `dateRange[end]` - End date (YYYY-MM-DD)
-- `obs_type` - Filter observations by: bugfix, feature, decision, discovery, change
+- `dateStart` - Start date (YYYY-MM-DD or epoch timestamp)
+- `dateEnd` - End date (YYYY-MM-DD or epoch timestamp)
+- `obs_type` - Filter observations by type (comma-separated): bugfix, feature, decision, discovery, change
 
 ## Examples
 
@@ -111,7 +111,7 @@ curl "http://localhost:37777/api/search?query=bug&type=observations&obs_type=bug
 
 **Find what happened last week:**
 ```bash
-curl "http://localhost:37777/api/search?query=&type=observations&dateRange[start]=2025-11-11&format=index&limit=10"
+curl "http://localhost:37777/api/search?query=&type=observations&dateStart=2025-11-11&format=index&limit=10"
 ```
 
 **Search everything:**