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/common-workflows.md

@@ -186,7 +186,7 @@ Then choose anchor manually.
 
 1. **Combine filters** for precision:
 ```bash
-curl -s "http://localhost:37777/api/search/observations?query=authentication&type=feature&dateRange[start]=2024-11-01&format=index&limit=10"
+curl -s "http://localhost:37777/api/search/observations?query=authentication&type=feature&dateStart=2024-11-01&format=index&limit=10"
 ```
 
 2. **Review filtered results**
@@ -207,7 +207,7 @@ Found 10 authentication features added in November:
 
 **Why this workflow:**
 - Multiple filters narrow results before requesting full details
-- Type + query + dateRange = precise targeting
+- Type + query + dateStart/dateEnd = precise targeting
 - Progressive disclosure: index first, full details selectively
 
 ---
@@ -228,7 +228,7 @@ Found 10 authentication features added in November:
 1. **Start with index format** - Always use `format=index` first
 2. **Use specialized tools** - by-type, by-file, by-concept when applicable
 3. **Compose operations** - Combine search + timeline for investigations
-4. **Filter early** - Use type, dateRange, project to narrow before expanding
+4. **Filter early** - Use type, dateStart/dateEnd, project to narrow before expanding
 5. **Progressive disclosure** - Load full details only for relevant items
 
 ## Token Budget Awareness