# Task ID: 48

**Title:** Implement begin_session tool for gated sessions

**Status:** done

**Dependencies:** 46 ✓, 47 ✓

**Priority:** high

**Description:** Create the begin_session MCP tool that accepts 5 keywords, triggers prompt selection, returns matched content with encouragement, and ungates the session.

**Details:**

1. Add begin_session tool definition in `/src/mcplocal/src/router.ts`:
   ```typescript
   private getBeginSessionTool(): Tool {
     return {
       name: 'begin_session',
       description: 'Start your session by providing 5 keywords that describe your current task. You\'ll receive relevant project context, policies, and guidelines. Required before using other tools.',
       inputSchema: {
         type: 'object',
         properties: {
           tags: {
             type: 'array',
             items: { type: 'string' },
             maxItems: 10,
             description: '5 keywords describing your current task'
           }
         },
         required: ['tags']
       }
     };
   }
   ```

2. Implement begin_session handler:
   - Validate tags array (1-10 items)
   - Call LlmPromptSelector or TagMatcher based on LLM availability
   - Fetch full content for selected prompts
   - Build response with matched content + index + encouragement
   - Ungate session
   - Send `notifications/tools/list_changed`

3. Response format:
   ```
   [Priority 10 prompts - full content]
   
   [Tag-matched prompts - full content, priority-ordered]
   
   Other prompts available that may become relevant...
   - name: summary
   ...
   If any seem related, request them with read_prompts({ tags: [...] }).
   ```

**Test Strategy:**

- Unit test: begin_session with valid tags returns matched prompts
- Unit test: begin_session includes priority 10 prompts always
- Unit test: begin_session response includes encouragement text
- Unit test: begin_session response includes prompt index
- Unit test: Session ungated after successful begin_session
- Unit test: notifications/tools/list_changed sent after ungating
- Unit test: Empty tags handled (returns priority 10 only)
- Unit test: Invalid tags rejected with error
- Unit test: begin_session while already ungated returns error