fix(delegate): wire model/provider params through to _build_child_agent

[Altairpaca]

The delegate_task() function already declared model and provider parameters in its signature (lines 1924-1925) and documented them in the docstring, but they were never actually passed through to _build_child_agent().

Three missing links added:

1. Single-task dict now includes model/provider so they reach the batch loop
2. Per-call model/provider override creds after credential resolution from config
3. Per-task model/provider resolution in the batch iteration loop

This lets callers do:
delegate_task(goal="...", model="deepseek-v4-pro")
to run a subagent on a specific model, with per-task overrides in batch mode. When unset, children inherit from parent (existing behaviour preserved).

What does this PR do?

Related Issue

Fixes #

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

How to Test

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:

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

For New Skills

• This skill is broadly useful to most users (if bundled) — see Contributing Guide
• SKILL.md follows the standard format (frontmatter, trigger conditions, steps, pitfalls)
• No external dependencies that aren't already available (prefer stdlib, curl, existing Hermes tools)
• I've tested the skill end-to-end: hermes --toolsets skills -q "Use the X skill to do Y"

Screenshots / Logs

[alt-glitch]

Duplicate of #3172 (earliest open PR in the delegate_task model/provider override cluster). Multiple open PRs implement the same feature: #6771, #12715, #12794, #16163, #17581, #17718, #17756, #19719, #23266, #30388. Please coordinate with those PRs before proceeding.

[Altairpaca]

Thanks for the pointer! #3172 by @ReqX is indeed the earliest in this cluster. Our commit is against the current
codebase (post-dispatch-refactor) and was a minimal wiring exercise. Since yours covers more ground (dispatch path fix, full test suite, skill template), I'm closing this in favour of #3172. Happy to rebase or contribute tests if that helps move things forward 🙏