Add py.typed marker, cap dependency versions, and validate task statu…

[ajaycj]

refactor: implement status validation and update dependencies

• src/switchplane/py.typed: Add PEP 561 marker for type checking.
• pyproject.toml: Update version caps for aiosqlite, langgraph, prompt_toolkit, structlog, langchain-core, and mcp.
• src/switchplane/persistence.py: Add _VALID_TRANSITIONS table and enforce validation in update_task.
• tests/test_persistence.py: Fix PENDING->COMPLETED transition and add TestStatusTransitions suite (6 tests).
• tests/test_control_plane.py: Fix PENDING->COMPLETED shortcut logic.

[salesforce-cla]

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[demianbrecht]

The dependency version caps and py.typed marker are fine. My concern is with the transition enforcement — see inline comment.

[demianbrecht]

This hardcodes a state machine directly in the persistence layer — every app built on Switchplane gets the same rigid transitions with no way to override or opt out.

Different apps may have legitimate reasons for different flows (e.g., synchronous tasks that skip RUNNING, custom intermediate states, domain-specific lifecycle rules). Transition policy is an application concern, not a framework storage concern.

If we want to offer transition validation as a convenience, it should be opt-in or configurable at the app/task level — not baked into Store.update_task() where it silently constrains every consumer.