fix: preserve goals across compression sessions

[denisqq]

Summary

• Add migrate_goal_to_session() so unfinished /goal state can be rebound from the compressed parent session to the continuation session.
• Invoke goal migration from the context compression session-rotation path after the new session row is created.
• Add a regression test proving the continuation session still sees the active goal after migration.

Closes #33618

Test plan

• python -m pytest tests/hermes_cli/test_goals.py tests/gateway/test_goal_verdict_send.py tests/gateway/test_goal_status_notice.py tests/gateway/test_goal_max_turns_config.py tests/tui_gateway/test_goal_command.py -q -o 'addopts='
• python -m pytest tests/run_agent/test_compression_persistence.py tests/run_agent/test_compression_boundary.py tests/run_agent/test_compression_boundary_hook.py tests/gateway/test_compression_session_id_persistence.py -q -o 'addopts='

[teknium1]

Merged via #49772 — your authorship is preserved as a co-author trailer on the squashed/rebased commit. This combined cluster fix incorporates the /goal-migrates-across-compression-rotation fix from this PR. Thanks for the contribution! #49772