#8274: feat(cron): add payload.fallbacks support for isolated jobs

## Summary - Adds `payload.fallbacks` field to `agentTurn` payloads for isolated cron jobs - Validates each fallback against the model catalog before execution - Payload fallbacks take priority over agent-level and global fallbacks - Empty array `[]` explicitly disables fallbacks (doesn't fall back to agent/global config) ## Alignment with Project Goals 🎯 This PR directly supports the **Performance** priority from the roadmap: > **Performance**: Optimizing token usage and compaction logic. With `payload.fallbacks`, users can now: - **Optimize token costs per job**: Use cheaper/faster models (like Haiku) as primary with expensive models as fallback only when needed - **Reduce latency**: Configure lightweight models for time-sensitive jobs without sacrificing reliability - **Fine-tune by use case**: Different jobs can have different fallback strategies without creating separate agents - **Avoid over-provisioning**: No need to default to expensive models "just in case" - fallbacks handle failures gracefully ## Motivation Currently, isolated jobs with `agentTurn` support `payload.model` for model override, but **do not support** `payload.fallbacks` for job-specific fallbacks. This forces users to create separate agents just to have different fallbacks, which is verbose and unnecessary. ## Changes - **`src/cron/types.ts`**: Added `fallbacks?: string[]` to `CronPayload` and `CronPayloadPatch` - **`src/cron/isolated-agent/run.ts`**: Extract, validate, and pass payload fallbacks to `runWithModelFallback()` - **`src/cron/isolated-agent.payload-fallbacks.test.ts`**: 6 tests covering all scenarios ## Fallback Priority (now) 1. `payload.fallbacks` (if defined) 2. `agents.list[agentId].model.fallbacks` 3. `agents.defaults.model.fallbacks` ## Example Usage ```json { "sessionTarget": "isolated", "payload": { "kind": "agentTurn", "message": "Generate portfolio report...", "model": "anthropic/claude-haiku-4-5-20251001", "fallbacks": ["opencode/kimi-k2.5-free", "anthropic/claude-sonnet-4-20250514"] } } ``` ## Test plan - [x] `payload.fallbacks` is correctly passed to `runWithModelFallback` - [x] Invalid fallbacks (not array) return error - [x] Invalid fallback entries (not strings) return error - [x] Disallowed models return error - [x] Empty `[]` disables fallbacks - [x] Undefined payload.fallbacks falls back to agent config - [x] All existing tests pass 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <noreply@anthropic.com>