fix: thread safety for concurrent subagent delegation

[teknium1]

Summary

Salvage of PR #1471 by @peteromallet — thread safety fixes for concurrent subagent delegation.

The problem

Running 3+ subagents concurrently via delegate_task in batch mode causes segfaults, data corruption, and intermittent crashes from four distinct race conditions.

Fixes

1. Remove redirect_stdout/redirect_stderr from delegate_tool
contextlib.redirect_stdout mutates the global sys.stdout. When multiple child agents start concurrently in a ThreadPoolExecutor, the race between redirect and the spinner thread corrupts the file descriptor, causing segfaults. The redirect was redundant — children already run with quiet_mode=True.

2. Split agent construction from execution
_run_single_child() → _build_child_agent() (main thread, serial) + _run_single_child() (worker thread, parallel). AIAgent construction creates httpx clients and initializes SSL contexts, which are not thread-safe to do concurrently.

3. Add threading.Lock to SessionDB
Subagents share the parent's SessionDB and call create_session(), append_message(), etc. from worker threads with no synchronization. Every database-accessing method is now wrapped in with self._lock:.

4. Add _active_children_lock to AIAgent
interrupt() iterates _active_children while worker threads append/remove children. Now copies the list under lock before iterating.

5. Add _client_cache_lock to auxiliary_client
Multiple subagent threads may resolve auxiliary clients concurrently via call_llm(). Double-checked locking pattern prevents duplicate client creation.

What was NOT included from the original PR

• Per-task model/provider overrides in delegate_task schema (feature addition, not a safety fix)
• resolve_provider_credentials() helper (utility, not needed for the safety fixes)
• _apply_provider_credentials() extraction in run_agent.py (refactoring, not a safety fix)

Files changed

File	Change
tools/delegate_tool.py	Split build/run, remove redirect, use lock
hermes_state.py	Add threading.Lock to all DB methods
run_agent.py	Add _active_children_lock, use in interrupt()
agent/auxiliary_client.py	Add _client_cache_lock, double-checked locking
6 test files	Update for new _run_single_child signature + add _active_children_lock

Tests

Full suite: 4911 passed, 8 pre-existing failures (unrelated), 200 skipped.

Credit

Original implementation by @peteromallet (PR #1471).
Closes #1471