fix(delegate): share credential pools with subagents

[MestreY0d4-Uninter]

Summary

This PR makes delegated child agents participate in credential-pool rotation instead of inheriting a single fixed credential from the parent.

Today, even when a parent agent has access to a healthy credential pool, a subagent can end up knowing only one effective credential. If that credential hits a rate limit or quota wall, the child stalls even though the pool still has other usable entries.

This patch keeps the change intentionally small:

• resolve an appropriate credential pool for the child
• share the parent's pool when parent and child use the same provider
• otherwise try to load the child's provider pool directly
• fall back gracefully to the old fixed-credential behavior when no pool exists
• add focused delegation tests for the child-pool resolution path

What changed

• added _resolve_child_credential_pool() in tools/delegate_tool.py
• _build_child_agent() now assigns a credential pool to the child when available
• same-provider children share the parent's pool so cooldown/rotation state stays synchronized
• different-provider children try to load their own provider pool
• empty or unavailable pools fall back to the previous behavior (no pool assignment)

Why

This is the smallest useful extraction from the larger credential-pool hardening branch.

It improves real subagent resilience without pulling in the more invasive pieces yet:

• no lease scheduler
• no tier profiles
• no retry-header parsing
• no fallback-precedence rewrite

Validation

Automated

• /home/ubuntu/.hermes/hermes-agent/venv/bin/python -m py_compile tools/delegate_tool.py tests/tools/test_delegate.py
• /home/ubuntu/.hermes/hermes-agent/venv/bin/python -m pytest -q -o addopts='' tests/tools/test_delegate.py -k 'ChildCredentialPoolResolution or DelegationProviderIntegration'
• Result: 13 passed, 44 deselected

Additional focused validation:

• /home/ubuntu/.hermes/hermes-agent/venv/bin/python -m pytest -q -o addopts='' tests/tools/test_delegate.py
• Result: 57 passed

Related context

• Extracted from the larger draft: [DRAFT] fix(credential-pool): harden multi-account pool against rate limits and subagent stampede #5374
• Related issue: Feature Request: Support multiple OAuth tokens with automatic fallback #933 (multiple OAuth tokens / automatic fallback)
• Intentionally does not implement proactive throttling from Track rate limit headers for proactive throttling #5449

Scope notes

This PR only makes child agents see and use credential pools.
It does not yet add:

• lease-based concurrency control
• tiered delegation profiles
• structured provider failure classification
• fallback precedence changes

[teknium1]

Closing — fully subsumed by #5580 (which includes credential sharing + leasing). Thanks @MestreY0d4-Uninter.