fix(delegation): keep child sessions out of parent env

[BROCCOLO1D]

Summary

• Prevent delegate_task child agents from overwriting the parent process's HERMES_SESSION_ID environment variable.
• Closes delegate_task subagent overwrites os.environ["HERMES_SESSION_ID"] in parent process #37356.

Why

• Child agents already have their own session_id for session logging, but mutating os.environ is process-global.
• After a delegate_task call, later parent tools can inherit the child's HERMES_SESSION_ID even though the parent agent's session_id is unchanged.

Changes

• gateway/session_context.py: add an update_environ switch to set_current_session_id while keeping the ContextVar update.
• agent/agent_init.py: use child-local ContextVar session IDs for agents with parent_session_id without changing process env.
• tests/run_agent/test_session_env.py: cover top-level env publication and subagent env preservation.

Validation

• python -m pytest tests/run_agent/test_session_env.py -q -o 'addopts='
• python -m pytest tests/gateway/test_session_env.py -q -o 'addopts='
• python -m ruff check gateway/session_context.py agent/agent_init.py tests/run_agent/test_session_env.py
• git diff --check

Scope

• In scope: avoiding process-global session env clobbering for child agents while preserving ContextVar session identity.
• Out of scope: migrating every existing os.environ reader to get_session_env().

[BROCCOLO1D]

Closing this as stale: there has been no activity or maintainer follow-up for over a week, and I don't want to leave inactive work open. Happy to reopen/rework if this becomes useful.