Sub-flow exit does not propagate to parent session

[nullhack]

Problem

When a sub-flow exits (e.g., review-gate-flow exits fail), the parent flow (development-flow) should handle the exit trigger and transition accordingly (e.g., review-gate: fail → tdd-cycle). However, the sub-flow exit does not propagate to the parent session.

Reproduction

1. Parent session at development-flow: review-gate, which calls review-gate-flow
2. Sub-flow transitions design-review → fail, exiting review-gate-flow as fail
3. Expected: parent session transitions review-gate → tdd-cycle (per development-flow.yaml next: fail: tdd-cycle)
4. Actual: parent session remains at review-gate. The fail exit is not handled.

Workaround

Manually set the parent session state:

python -m flowr session set-state tdd-cycle

Then edit the session YAML to update the state field.

Expected Behavior

When a sub-flow exits, flowr should:

1. Pop the sub-flow from the call stack
2. Apply the exit trigger to the parent flow's next mapping for the calling state
3. Transition the parent session to the target state

Environment

• flowr 0.4.x
• Python 3.14
• Session YAML with stack field containing parent flow/state