fix(orchestrator): guard missing plan blocks

[Gujiassh]

Summary

Fixes #261.

Guards the orchestrator chain-plan parser when the model output omits the <plan>...</plan> block, and aligns TaskRecorder.input with its declared str | list[TResponseInputItem] type.

Changes

• treat a missing <plan>...</plan> block as an empty plan payload instead of calling .group(1) on None
• preserve the existing assert len(tasks) > 0, "No tasks parsed from plan" failure so malformed plan output reports a clear assertion instead of an AttributeError
• change TaskRecorder.input from default_factory=dict to default_factory=list
• add focused regressions for the missing-plan path, the valid-plan path, and the TaskRecorder.input default type

Verification

• source .venv312/bin/activate && python -m pytest tests/agents/test_orchestrator_chain_regressions.py -q
• source .venv312/bin/activate && python -m ruff format utu/agents/common.py utu/agents/orchestrator/chain.py tests/agents/test_orchestrator_chain_regressions.py
• source .venv312/bin/activate && python -m ruff check utu/agents/common.py utu/agents/orchestrator/chain.py tests/agents/test_orchestrator_chain_regressions.py
• git diff --check