Add session-aware task context and auto-linking (CF-166 Phase 2)

Enhancements:

1. Session Context Table
   - Migration 014: session_context table
   - Tracks current working task per session
   - Auto-updated on task status changes

2. Auto-Linking to Current Working Task
   - When task status → in_progress: sets as current_task_id
   - When task status → completed: clears current_task_id
   - New tasks auto-link to current working task with 'relates_to'
   - Shows auto-link message during task creation

3. Session ID Resolution
   - Fixed path: ~/.cache/session-memory/current_session
   - Falls back to environment variable CLAUDE_SESSION_ID
   - Generates timestamp-based ID if unavailable

Example Flow:
```
1. task update CF-123 --status in_progress
   → Sets CF-123 as current working task

2. task add --title "Related work"
   → Created: CF-124
   🔗 Auto-linked to: CF-123 (current working task)

3. task update CF-123 --status completed
   → Clears current working task
```

Files Changed:
- migrations/014_session_context.sql (new, 33 lines)
- src/tools/crud.ts (auto-linking logic, session context management)

Benefits:
- Zero manual linking for related work
- Session context preserved automatically
- Investigation workflows auto-organized
- Retroactive work tracking prevented

Note: Requires MCP server restart to take effect.

Remaining CF-166 Features (Phase 3):
- task investigate command
- Investigation workflow helper
- Task creation reminders

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

migrations/014_session_context.sql

@@ -0,0 +1,28 @@
+-- Migration 014: Session Context for Auto-linking
+-- Tracks current working task per session for automatic task relationship creation
+
+CREATE TABLE IF NOT EXISTS session_context (
+    session_id TEXT PRIMARY KEY,
+    current_task_id TEXT REFERENCES tasks(id) ON DELETE SET NULL,
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_session_context_task ON session_context(current_task_id);
+
+-- Auto-update timestamp on changes
+CREATE OR REPLACE FUNCTION update_session_context_timestamp()
+RETURNS TRIGGER AS $$
+BEGIN
+    NEW.updated_at = NOW();
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER session_context_updated
+    BEFORE UPDATE ON session_context
+    FOR EACH ROW
+    EXECUTE FUNCTION update_session_context_timestamp();
+
+-- Record migration
+INSERT INTO schema_migrations (version) VALUES ('014_session_context')
+ON CONFLICT (version) DO NOTHING;