New modules:
- agent/flowgraph.py: FlowgraphManager subprocess lifecycle (start/stop/
  restart with SIGTERM→SIGKILL fallback) (#74)

Streaming pipeline:
- Agent handles cmd/stream topics: START spawns flowgraph + ZMQ-to-MQTT
  frame bridge, STOP tears down cleanly (#75)
- Agent CLI: added --sdr-device and --zmq-port arguments (#76)
- Director emits StreamCommand on AOS (with transmitter frequency,
  modulation, baud) and LOS (#77, #78)
- Full frame pipeline: Director triggers → Agent flowgraph → ZMQ →
  MQTT → Stream Router → UDP to YAMCS

Dependencies:
- Added pyzmq>=25.0 to agent requirements and pyproject.toml

Testing (9 new tests, 358 total):
- FlowgraphManager unit tests (start/stop/restart/force-kill/errors)
- Import check for agent.flowgraph module

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>