fix(agent) SWE coding-agent RL stability bugs (abort handling, session cleanup) - #2124

Port of gitlab MR !514 (subset present in the MR diff):

- generate.py: an aborted sample now sets rollout_log_probs=[0.0] and
  remove_sample=True so it contributes no gradient and is excluded from the
  GRPO/GSPO group baseline; finally now calls drop_session instead of
  finish_session.
- adapters/common.py: add drop_session() for cleanup-only teardown; make
  finish_session's base_sample a required arg.
- harness/common.py: run_command tolerates an empty done-marker (truncate-then
  -write window) instead of crashing on int('').
- trajectory.py: add TrajectoryManager.drop_session() to drop trees/turn-count
  idempotently.
- run_qwen36_35b_a3b_swe_8nodes.sh: switch algo args to grpo eps-clip 0.2/0.28.