feat: implement L1-L7 context tree for multi-layered memory management

Implements Pillar 2 (Multi-layered Context Management) with a 7-tier
hierarchical memory system from real-time market data to generational
trading wisdom.

## New Modules
- `src/context/layer.py`: ContextLayer enum and metadata config
- `src/context/store.py`: ContextStore for CRUD operations
- `src/context/aggregator.py`: Bottom-up aggregation (L7→L6→...→L1)

## Database Changes
- Added `contexts` table for hierarchical data storage
- Added `context_metadata` table for layer configuration
- Indexed by layer, timeframe, and updated_at for fast queries

## Context Layers
- L1 (Legacy): Cumulative wisdom (kept forever)
- L2 (Annual): Yearly metrics (10 years retention)
- L3 (Quarterly): Strategy pivots (3 years)
- L4 (Monthly): Portfolio rebalancing (2 years)
- L5 (Weekly): Stock selection (1 year)
- L6 (Daily): Trade logs (90 days)
- L7 (Real-time): Live market data (7 days)

## Tests
- 18 new tests in `tests/test_context.py`
- 100% coverage on context modules
- All 72 tests passing (54 existing + 18 new)

## Documentation
- Added `docs/context-tree.md` with comprehensive guide
- Updated `CLAUDE.md` architecture section
- Includes usage examples and best practices

Closes #15

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

src/db.py

@@ -39,6 +39,38 @@ def init_db(db_path: str) -> sqlite3.Connection:
     if "exchange_code" not in columns:
         conn.execute("ALTER TABLE trades ADD COLUMN exchange_code TEXT DEFAULT 'KRX'")
 
+    # Context tree tables for multi-layered memory management
+    conn.execute(
+        """
+        CREATE TABLE IF NOT EXISTS contexts (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            layer TEXT NOT NULL,
+            timeframe TEXT NOT NULL,
+            key TEXT NOT NULL,
+            value TEXT NOT NULL,
+            created_at TEXT NOT NULL,
+            updated_at TEXT NOT NULL,
+            UNIQUE(layer, timeframe, key)
+        )
+        """
+    )
+
+    conn.execute(
+        """
+        CREATE TABLE IF NOT EXISTS context_metadata (
+            layer TEXT PRIMARY KEY,
+            description TEXT NOT NULL,
+            retention_days INTEGER,
+            aggregation_source TEXT
+        )
+        """
+    )
+
+    # Create indices for efficient context queries
+    conn.execute("CREATE INDEX IF NOT EXISTS idx_contexts_layer ON contexts(layer)")
+    conn.execute("CREATE INDEX IF NOT EXISTS idx_contexts_timeframe ON contexts(timeframe)")
+    conn.execute("CREATE INDEX IF NOT EXISTS idx_contexts_updated ON contexts(updated_at)")
+
     conn.commit()
     return conn