fix(delegation): preserve parent session identity across child runs

[Xipong]

Summary

Delegated child construction currently calls set_current_session_id(child.session_id), which publishes the child ID into the process-wide os.environ["HERMES_SESSION_ID"]. The parent continues running in the same process, so later parent terminal/execute-code subprocesses can inherit the last child's identity.

This PR:

• suppresses the process-wide environment write while inside delegated_child_context();
• scopes the child ContextVar to construction/execution and restores the parent value on exit;
• explicitly binds child.session_id during the child run, so child terminal subprocesses still receive the correct identity through the existing subprocess-environment bridge;
• preserves the existing root CLI/gateway/cron behavior, including the exceptional fallback path.

No new configuration, schema, model-facing tools, or persistence behavior is introduced.

Reproduction

1. Start a parent agent session.
2. Dispatch one or more delegate_task children.
3. Run a terminal command from the parent and inspect HERMES_SESSION_ID.
4. Before this fix, the environment ID resolves in state.db to a source='subagent' session whose parent_session_id is the real parent.
5. With this fix, parent subprocesses retain the parent ID; each child subprocess receives only its own child ID through the task-local bridge.

Verification

• New single/nested/concurrent ContextVar + environment + real build_subprocess_env contracts: 7 passed
• Expanded session-context, local-env, API-server-background, single/batch delegation, Kanban/toolset/composite/summary/timeout matrix: 99 passed
• Focused fallback-neighbor matrix after final hardening: 19 passed
• Ruff and py_compile: passed

Current head: 56b8f3417521c627db605ed4c1060435a08b4ce1.

[teknium1]

Thanks for the focused fix. The premise is confirmed on current main: tools/delegate_tool.py:1503 constructs a child under delegated_child_context(), but agent/agent_init.py:1466 calls set_current_session_id, whose current implementation writes the child ID to process-wide os.environ at gateway/session_context.py:154. The current delegation context at agent/delegation_context.py:34-40 only tracks the child marker and does not restore the parent session ContextVar.

The proposed construction boundary, explicit child execution binding at tools/delegate_tool.py:2179, and subprocess-environment assertions align with the existing ContextVar bridge in tools/environments/local.py:635.

Automated hermes-sweeper review.

[teknium1]

Merged via #75856 — your commit(s) were cherry-picked onto current main with your authorship preserved in git history (rebase merge). Thanks for the contribution!