refactor: introduce AgentTriggerReason type union as SSOT for error reason strings

Replace scattered reason string literals with a centralized type hierarchy:
- AgentTriggerReason: all possible trigger reasons
- ClaudeRotationReason, CodexRotationReason, NoFallbackCooldownReason:
  derived subtypes for specific contexts
- AgentErrorClassification, FallbackTriggerResult, CodexRotationTriggerResult:
  discriminated unions for compile-time narrowing

Remove dead export isUsageExhausted() (superseded by
isPrimaryNoFallbackCooldownActive). Replace NO_FALLBACK_COOLDOWN_REASONS
Set with isNoFallbackCooldownReason() type guard.

Add tests for agent-error-detection and provider-retry (423 total passing).

Known limitation: 6 `as CodexRotationReason` casts in Codex rotation paths
where streamed trigger reason is AgentTriggerReason but runtime value is
always CodexRotationReason-compatible.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/message-agent-executor.test.ts

@@ -57,7 +57,6 @@ vi.mock('./provider-fallback.js', () => ({
   hasGroupProviderOverride: vi.fn(() => false),
   isFallbackEnabled: vi.fn(() => true),
   isPrimaryNoFallbackCooldownActive: vi.fn(() => false),
-  isUsageExhausted: vi.fn(() => false),
   markPrimaryCooldown: vi.fn(),
 }));