What does this PR do?

Fixes a delegation wiring gap: delegate_task() could only route child models
through delegation.model config or by inheriting the parent model. A single
delegation call could not explicitly choose a child model, and batch mode could
not route different sub-tasks to different models.

This PR adds per-call and per-task model overrides for delegation without
expanding the provider/credential system.

Model selection precedence is now:

• tasks[].model
• delegate_task(model=...)
• delegation.model
• parent_agent.model

It also keeps explicit model overrides from silently inheriting the parent's
fallback model chain, so a child that was explicitly routed to another model
does not fail back onto the parent model behind the caller's back.

This PR intentionally does not add per-task provider overrides. Provider,
base URL, API mode, and credential resolution remain controlled by existing
delegation config and parent runtime state.

Related Issue

Fixes the delegate_task per-call / per-task model override gap described in
the delegation model routing discussion.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✅ Tests (adding or improving test coverage)

Changes Made

• Added top-level model support to tools/delegate_tool.py
delegate_task(...).
• Added per-task tasks[].model support to the delegate tool schema.
• Threaded model override precedence through child construction:
  • tasks[].model > delegate_task(model=...) > delegation.model > parent.model
• Added _normalize_model_override(...) helper to keep override parsing narrow
  and explicit.
• Prevented explicit model overrides from inheriting the parent's fallback chain.
• Added regression coverage in tests/tools/test_delegate.py for:
  • schema exposure of model
  • top-level model override
  • per-task model override beating top-level
  • per-call override beating delegation.model
  • config model still applying without override
  • explicit model override clearing inherited fallback

How to Test

1. Run the delegate tool test suite:

   python3 -m pytest -q -n 0 tests/tools/test_delegate.py

2. Confirm the suite passes.
3. Verify the new behaviors covered by tests:
   • schema exposes top-level model and tasks[].model
   • single-call override reaches child AIAgent(model=...)
   • tasks[].model beats top-level model
   • explicit override beats delegation.model
   • no override keeps current delegation.model behavior
   • explicit override does not inherit parent fallback chain

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: WSL / Linux python3 -m pytest

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Screenshots / Logs

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