#23738: feat(fallback): first-class transition visibility + low-noise automated notices

## Summary This PR adds first-class visibility when model fallback begins, plus optional low-noise notices for automated contexts. ### What changed 1. **Transition-start signal/event (`primary -> fallback`)** - Added `onTransition` callback support to `runWithModelFallback(...)`. - Emits new lifecycle phase: `fallback_start` when transitioning to the next model candidate (including primary -> first fallback). - Wired into: - `runAgentTurnWithFallback` (auto-reply runtime events) - `agent` command execution path (lifecycle + runtime log line) - isolated cron agent runs (lifecycle + warn log) 2. **Surfaced in runtime/status context** - Fallback state now tracks `fallbackNoticeStartedAt` in session state. - Status output now shows fallback duration (`since ...`) when active. 3. **Optional low-noise automated notify behavior (deduped)** - New config flag: `agents.defaults.modelFallbackNotifyAutomated` (default: disabled). - When enabled: - heartbeat/automated reply flow emits a compact `Model Fallback started` notice **once per transition** - isolated cron flow prepends/emits the same compact notice **once per transition** - No repeated notice spam while fallback remains active. 4. **Transition + dedupe tests** - Added tests for: - transition callback emission in model fallback core - cooldown-triggered transition emission - `fallback_start` lifecycle emission - heartbeat automated notice dedupe - cron automated notice dedupe - fallback start timestamp tracking/preservation - status fallback duration rendering 5. **Docs/config references** - Updated failover docs with `fallback_start` lifecycle semantics and automated notify behavior. - Added config label/help/schema entries for `agents.defaults.modelFallbackNotifyAutomated`. ## Backward compatibility - Default behavior is preserved unless `agents.defaults.modelFallbackNotifyAutomated` is explicitly enabled. - Existing `fallback` / `fallback_cleared` behavior remains intact. ## Validation - `pnpm exec tsc -p tsconfig.json --noEmit` - `pnpm vitest run --config vitest.unit.config.ts src/agents/model-fallback.test.ts src/auto-reply/fallback-state.test.ts src/auto-reply/status.test.ts src/auto-reply/reply/agent-runner.runreplyagent.test.ts src/cron/isolated-agent/run.skill-filter.test.ts` <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR adds comprehensive fallback transition visibility to OpenClaw's model failover system. The implementation introduces a new `onTransition` callback mechanism in the core fallback runner that fires when switching between model candidates, emits lifecycle events (`fallback_start`, `fallback`, `fallback_cleared`) at key transition points, and tracks fallback state with timestamps in session storage. Key additions include: - **Transition callbacks**: `ModelFallbackTransitionHandler` type and `onTransition` parameter wired into auto-reply, CLI agent command, and isolated cron flows - **Fallback state tracking**: New session fields (`fallbackNoticeSelectedModel`, `fallbackNoticeActiveModel`, `fallbackNoticeReason`, `fallbackNoticeStartedAt`) for deduped notice management - **Status output enhancement**: Fallback duration display showing "since X ago" when fallback is active - **Optional automated notices**: New `modelFallbackNotifyAutomated` config flag enabling compact one-time transition notices for heartbeat/cron contexts - **Comprehensive test coverage**: 103 passing tests across model-fallback, fallback-state, agent-runner, status, and cron test suites The implementation maintains backward compatibility (default behavior unchanged unless `modelFallbackNotifyAutomated` is enabled) and follows existing patterns for lifecycle event emission and state management. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no blocking issues identified - The implementation is well-architected with proper typing, comprehensive test coverage (103 tests passing), and maintains backward compatibility. The transition callback mechanism follows TypeScript best practices with proper async/await handling, the deduplication logic correctly prevents notification spam by tracking state transitions, and all integration points (auto-reply, CLI commands, cron) are consistently wired. The changes are thoroughly documented and align with existing codebase patterns. - No files require special attention _{Last reviewed commit: ae5e779} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->