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>

src/servers/search-server.ts

@@ -346,9 +346,9 @@ const filterSchema = z.object({
 const tools = [
   {
     name: 'search_observations',
-    description: 'Search observations using full-text search across titles, narratives, facts, and concepts. IMPORTANT: Always use index format first (default) to get an overview with minimal token usage, then use format: "full" only for specific items of interest.',
+    description: 'Search observations using hybrid semantic + full-text search (ChromaDB primary, SQLite FTS5 fallback). IMPORTANT: Always use index format first (default) to get an overview with minimal token usage, then use format: "full" only for specific items of interest.',
     inputSchema: z.object({
-      query: z.string().describe('Search query for FTS5 full-text search'),
+      query: z.string().describe('Natural language search query (semantic ranking via ChromaDB, FTS5 fallback)'),
       format: z.enum(['index', 'full']).default('index').describe('Output format: "index" for titles/dates only (default, RECOMMENDED for initial search), "full" for complete details (use only after reviewing index results)'),
       ...filterSchema.shape
     }),
@@ -434,9 +434,9 @@ const tools = [
   },
   {
     name: 'search_sessions',
-    description: 'Search session summaries using full-text search across requests, completions, learnings, and notes. IMPORTANT: Always use index format first (default) to get an overview with minimal token usage, then use format: "full" only for specific items of interest.',
+    description: 'Search session summaries using hybrid semantic + full-text search (ChromaDB primary, SQLite FTS5 fallback). IMPORTANT: Always use index format first (default) to get an overview with minimal token usage, then use format: "full" only for specific items of interest.',
     inputSchema: z.object({
-      query: z.string().describe('Search query for FTS5 full-text search'),
+      query: z.string().describe('Natural language search query (semantic ranking via ChromaDB, FTS5 fallback)'),
       format: z.enum(['index', 'full']).default('index').describe('Output format: "index" for titles/dates only (default, RECOMMENDED for initial search), "full" for complete details (use only after reviewing index results)'),
       project: z.string().optional().describe('Filter by project name'),
       dateRange: z.object({
@@ -1000,9 +1000,9 @@ const tools = [
   },
   {
     name: 'search_user_prompts',
-    description: 'Search raw user prompts with full-text search. Use this to find what the user actually said/requested across all sessions. IMPORTANT: Always use index format first (default) to get an overview with minimal token usage, then use format: "full" only for specific items of interest.',
+    description: 'Search raw user prompts using hybrid semantic + full-text search (ChromaDB primary, SQLite FTS5 fallback). Use this to find what the user actually said/requested across all sessions. IMPORTANT: Always use index format first (default) to get an overview with minimal token usage, then use format: "full" only for specific items of interest.',
     inputSchema: z.object({
-      query: z.string().describe('Search query for FTS5 full-text search'),
+      query: z.string().describe('Natural language search query (semantic ranking via ChromaDB, FTS5 fallback)'),
       format: z.enum(['index', 'full']).default('index').describe('Output format: "index" for truncated prompts/dates (default, RECOMMENDED for initial search), "full" for complete prompt text (use only after reviewing index results)'),
       project: z.string().optional().describe('Filter by project name'),
       dateRange: z.object({