feat: RuntimeState event bus integration with checkpoint/resume

[greysonlalonde]

Summary

• Pass RuntimeState as optional third arg to event bus handlers
• RuntimeState.checkpoint(dir) writes timestamped JSON snapshots
• Crew.from_checkpoint(path) restores and resumes via kickoff()
• _get_execution_start_index skips tasks with existing output
• Convert CrewStructuredTool, StandardPromptResult, SystemPromptResult, TokenCalcHandler to BaseModel
• CrewAgentExecutorMixin uses Field(exclude=True) for back-references

Test plan

• Real LLM execution: checkpoint after task 1, restore, resume skips task 1 and runs task 2
• 371 core tests pass
• Backwards compatible: 2-arg event handlers still work

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Note

High Risk
High risk because it changes core execution flow (agent executors, task skipping/resume) and event emission semantics by introducing shared RuntimeState recording and passing it into handlers.

Overview
Adds first-class checkpoint/resume by serializing a unified RuntimeState (entities + event record) to timestamped JSON and restoring Crew, Flow, and Agent via new from_checkpoint() APIs that rehydrate runtime links, rebuild event scope, and resume execution from the first incomplete task.

Integrates RuntimeState into the event system: the event bus now records emitted events, auto-registers emitting entities, and optionally passes the current runtime state as a third argument to sync/async handlers while remaining compatible with existing 2-arg handlers.

Refactors executor/state serialization to support checkpointing: introduces BaseAgentExecutor (Pydantic model) as a shared base for CrewAgentExecutor and AgentExecutor, adds resumable message handling, and updates LLM/executor fields to round-trip as structured dicts (with llm_type/executor_type discriminators) across agents/crews.

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[iris-clawd]

Approved. RuntimeState checkpoint/resume with event record, serializable executors/tools/LLMs, and Flow subclass restoration all look solid after multiple review rounds. Minor follow-ups (EventRecord memory growth for long runs, lazy event type map) can be addressed separately.