fix(gateway): add /reasoning --session override

[irispillars]

Summary

• Adds /reasoning <level> --session for session/thread-scoped reasoning effort overrides without writing config.yaml
• Adds /reasoning reset --session to clear a session override and fall back to global config
• Routes normal agent runs, /background, and /btw through the session-aware reasoning resolver
• Clears session reasoning overrides on /new and compression auto-reset

Test Plan

• python -m py_compile gateway/run.py
• scripts/run_tests.sh tests/gateway/test_reasoning_command.py tests/gateway/test_session_model_reset.py tests/gateway/test_running_agent_session_toggles.py -q

Closes #9586

[alt-glitch]

Competing PRs for #9586: also see #14558 (session-scoped by default) and #15522 (duplicate of #14558). This PR takes the --session flag approach instead of making session-scoped the default.

[alt-glitch]

Competing PRs for #9586: also see #14558 and #15522.

[irispillars]

Closing per requester preference. Reworking the design so is session-scoped by default and is the explicit persistent/global path.