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/manager.ts

@@ -6,6 +6,7 @@ export interface Session {
   addMessage(message: Message): void;
   getHistory(): Message[];
   clear(): void;
+  replaceHistory(messages: Message[]): void;
 }
 
 export class ManagedSession implements Session {
@@ -34,6 +35,16 @@ export class ManagedSession implements Session {
     this.store.clearSession(this.id);
   }
 
+  /**
+   * Replace the entire session history with new messages.
+   * Used after compaction to persist the compacted state.
+   * Updates both in-memory history and SQLite storage atomically.
+   */
+  replaceHistory(messages: Message[]): void {
+    this.history = [...messages];
+    this.store.replaceMessages(this.id, messages);
+  }
+
   setHistory(messages: Message[]): void {
     this.history = [...messages];
   }