fix: validate delegated sub-agent tool lists resolve to >=1 tool (Closes #1387)

[Lexus2016]

Automated evolution PR for issue #1387.

Problem

When a delegated sub-agent's toolset resolution produced an empty list (requested toolsets don't intersect with parent's, or inherited toolsets are all stripped by _strip_blocked_tools), the sub-agent was launched with zero tools. This caused the sub-agent to spiral on every tool call — the root cause of the 'is not a deferrable tool' pattern observed in introspection (64 failed sessions, 4.14% rate).

Fix

Added a validation guard in _build_child_agent after toolset resolution:

• If child_toolsets is empty AND toolsets were explicitly requested → ValueError naming the requested toolsets and parent's available toolsets
• If child_toolsets is empty from inherited toolsets → ValueError explaining the parent has no surviving enabled toolsets
• The caller in delegate_task catches the ValueError and returns a per-task error result with status: "error" instead of launching the toolless sub-agent

Changes

• tools/delegate_tool.py: Empty-toolset guard in _build_child_agent (raises ValueError); try/except in delegate_task batch loop catches it and returns structured error result
• tests/tools/test_delegate_toolset_scope.py: Added TestEmptyToolsetValidation with 3 tests covering explicit-request-empty, inherited-empty, and valid-toolsets-no-raise

Validation

• Lint: ✓ (ruff check)
• Tests: 14/14 passed (tests/tools/test_delegate_toolset_scope.py)

Co-Authored-By: Hermes Evolution evolution@hermes.ai

[Lexus2016]

Blocked by code-review: CI failing — the validation guard in _build_child_agent causes IndexError at delegate_tool.py:3240, breaking 8 tests in test_delegate_shallow_retry.py. The guard raises ValueError which is caught per-task, but the return-path change at line 3240 accesses an index that no longer exists in the modified control flow. Needs: fix the IndexError so existing shallow-retry tests pass, or restructure the guard to not alter the function's return-path shape.