fix(memory): restore flush memory persistence

[bobashopcashier]

Summary

• enable built-in memory for the gateway pre-reset flush agent so memory tool calls can persist writes
• honor auxiliary.flush_memories.timeout in flush_memories() for both the auxiliary path and the direct chat-completions fallback
• add focused regressions for gateway flush agent construction and timeout propagation

Issues Addressed

• [Bug]: Gateway memory flush creates agent with skip_memory=True, making memory tool non-functional #6157
• [Bug]: flush_memories() hardcodes 30s timeout, ignores config value #6154

Related PR Overlap

• This PR subsumes the gateway-only #6157 fix proposed in fix(gateway): init memory store for flush agent #6179.
• This PR also subsumes the timeout-only #6154 fix proposed in fix(agent): respect config timeout for flush_memories #6177.
• The branch intentionally combines both flush-path fixes into one focused change because they affect the same memory flush correctness surface.

Why

The gateway flush path was constructing a temporary agent with memory disabled, which made memory tool calls fail during pre-reset flushes. Separately, the flush helper hardcoded a 30 second timeout and ignored the configured auxiliary timeout.

Testing

• source .venv/bin/activate && python -m pytest tests/run_agent/test_flush_memories_codex.py tests/gateway/test_flush_memory_stale_guard.py -q

Notes

• This PR is intentionally scoped to the memory flush correctness path only.
• Separate restart/dedup flush issues are not included here.