feat: add multi-model delegation (Phase 0) and context compaction (Phase 1)

Phase 0 — Multi-Model Delegation:
- AgentOrchestrator wraps NativeAgent with delegate() for stateless
  single-turn calls to any model tier (fast/default/complex/local)
- DelegationConfig maps task types (compaction, classification, etc.)
  to model tiers
- Delegation prompts for compaction, memory extraction, classification,
  and tool summarisation
- Per-tier usage tracking for cost visibility
- Config schema: agents.delegation and agents.primary_tier

Phase 1 — Context Compaction:
- Token estimation (char/4 heuristic) with context window lookup
- shouldCompact() threshold check against context window percentage
- compactHistory() splits old/recent messages, delegates summary to
  fast tier, returns CompactionResult
- Automatic compaction in AgentOrchestrator.process() when configured
- Force-compact via orchestrator.compact() with session persistence
- Session.replaceHistory() with atomic SQLite transaction
- /compact TUI command with feedback on compacted token counts
- Config schema: compaction.enabled, threshold_pct, keep_turns,
  summary_max_tokens

Tests: 385 passing across 50 files (22 new tests in 2 new test files)

src/session/store.ts

@@ -40,6 +40,26 @@ export class SessionStore {
     }));
   }
 
+  /**
+   * Atomically replace all messages for a session.
+   * Used by compaction to swap full history with a compacted version.
+   * Runs in a transaction: delete all → re-insert in order.
+   */
+  replaceMessages(sessionId: string, messages: Message[]): void {
+    const transaction = this.db.transaction(() => {
+      // Delete existing messages
+      this.db.prepare('DELETE FROM messages WHERE session_id = ?').run(sessionId);
+      // Re-insert in order
+      const insert = this.db.prepare(
+        'INSERT INTO messages (session_id, role, content) VALUES (?, ?, ?)'
+      );
+      for (const msg of messages) {
+        insert.run(sessionId, msg.role, msg.content);
+      }
+    });
+    transaction();
+  }
+
   clearSession(sessionId: string): void {
     const stmt = this.db.prepare('DELETE FROM messages WHERE session_id = ?');
     stmt.run(sessionId);