docs: Align search documentation with hybrid ChromaDB architecture (#116)

* feat: Add discovery_tokens for ROI tracking in observations and session summaries

- Introduced `discovery_tokens` column in `observations` and `session_summaries` tables to track token costs associated with discovering and creating each observation and summary.
- Updated relevant services and hooks to calculate and display ROI metrics based on discovery tokens.
- Enhanced context economics reporting to include savings from reusing previous observations.
- Implemented migration to ensure the new column is added to existing tables.
- Adjusted data models and sync processes to accommodate the new `discovery_tokens` field.

* refactor: streamline context hook by removing unused functions and updating terminology

- Removed the estimateTokens and getObservations helper functions as they were not utilized.
- Updated the legend and output messages to replace "discovery" with "work" for clarity.
- Changed the emoji representation for different observation types to better reflect their purpose.
- Enhanced output formatting for improved readability and understanding of token usage.

* Refactor user-message-hook and context-hook for improved clarity and functionality

- Updated user-message-hook.js to enhance error messaging and improve variable naming for clarity.
- Modified context-hook.ts to include a new column key section, improved context index instructions, and added emoji icons for observation types.
- Adjusted footer messages in context-hook.ts to emphasize token savings and access to past research.
- Changed user-message-hook.ts to update the feedback and support message for clarity.

* fix: Critical ROI tracking fixes from PR review

Addresses critical findings from PR #111 review:

1. **Fixed incorrect discovery token calculation** (src/services/worker/SDKAgent.ts)
   - Changed from passing cumulative total to per-response delta
   - Now correctly tracks token cost for each observation/summary
   - Captures token state before/after response processing
   - Prevents all observations getting inflated cumulative values

2. **Fixed schema version mismatch** (src/services/sqlite/SessionStore.ts)
   - Changed ensureDiscoveryTokensColumn() from version 11 to version 7
   - Now matches migration007 definition in migrations.ts
   - Ensures consistent version tracking across migration system

These fixes ensure ROI metrics accurately reflect token costs.

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

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

* fix: Update search documentation to reflect hybrid ChromaDB architecture

The backend correctly implements ChromaDB-first semantic search with SQLite
temporal ordering and FTS5 fallback, but documentation incorrectly described
it as "FTS5 full-text search". This fix aligns all skill guides and tool
descriptions with the actual implementation.

Changes:
- Update SKILL.md to describe hybrid architecture with ChromaDB primary
- Update observations.md title and query parameter descriptions
- Update all three search tool descriptions in search-server.ts:
  * search_observations
  * search_sessions
  * search_user_prompts

All tools now correctly document:
- ChromaDB semantic search (primary ranking)
- 90-day recency filter
- SQLite temporal ordering
- FTS5 fallback (when ChromaDB unavailable)

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

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

* fix: Add discovery_tokens column to observations and session_summaries tables

---------

Co-authored-by: Claude <noreply@anthropic.com>

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

@@ -1,4 +1,4 @@
-# Search Observations (Full-Text)
+# Search Observations (Semantic + Full-Text Hybrid)
 
 Search all observations using natural language queries.
 
@@ -17,7 +17,7 @@ curl -s "http://localhost:37777/api/search/observations?query=authentication&for
 
 ## Parameters
 
-- **query** (required): Search terms (e.g., "authentication", "bug fix", "database migration")
+- **query** (required): Natural language search query - uses semantic search (ChromaDB) for ranking with SQLite FTS5 fallback (e.g., "authentication", "bug fix", "database migration")
 - **format**: "index" (summary) or "full" (complete details). Default: "full"
 - **limit**: Number of results (default: 20, max: 100)
 - **project**: Filter by project name (optional)