Support `strategy.matrix` on the `agent` job for parallel AI execution

[chepa92]

Problem

Currently, the strategy / matrix support in gh-aw only applies to custom jobs defined under jobs: in the frontmatter (e.g., the BatchOps pattern). The agent job itself — the core AI execution job — does not support matrix strategy, meaning it always runs as a single instance per workflow run.

This is a limitation for use cases like randomized testing at scale, where you want to dispatch N parallel agent threads with different parameters (random seeds, test configs, shards) and have each agent instance work independently and concurrently.

Use Case

I want to run randomized test campaigns where:

1. Multiple agent instances run in parallel as matrix jobs (e.g., 5–10 concurrent threads)
2. Each instance receives different matrix parameters (e.g., seed, test-suite, scenario)
3. Each agent independently generates and executes randomized test scenarios using its own AI session
4. Results from all parallel agents are aggregated at the end

Today, achieving this requires either:

• Running agents sequentially (slow, defeats the purpose)
• Using workflow_dispatch fan-out with multiple workflow runs (wasteful, hard to coordinate, hits per-engine concurrency)
• Custom jobs with matrix that call back into agent workflows (fragile, not a first-class pattern)

Proposed Solution

Allow strategy on the agent job, either at the top level or under engine:

---
on:
  workflow_dispatch:
    inputs:
      threads:
        default: "5"

strategy:
  matrix:
    thread: [0, 1, 2, 3, 4]
  fail-fast: false
  max-parallel: 5

engine:
  id: copilot
  concurrency:
    group: "gh-aw-copilot-${{ github.workflow }}-${{ matrix.thread }}"

tools:
  bash: ["pytest", "jq"]
---

# Randomized Test Runner — Thread ${{ matrix.thread }}

Run randomized test suite using seed derived from thread index ${{ matrix.thread }}.
Each thread independently selects and executes a random subset of tests.

Key requirements:

• Matrix parameters available in markdown body via ${{ matrix.* }} expressions
• Per-instance concurrency groups so parallel agent instances don't cancel each other
• fail-fast: false support so one failing agent doesn't kill the others
• max-parallel to cap concurrent AI sessions and manage token/API costs

Prior Art

• Issue [plan] Fix or remove unusable strategy schema definition for custom jobs #19402 removed the unusable strategy schema from custom jobs (was an empty-object-only definition). The BatchOps pattern now shows working matrix on custom jobs, but not on the agent job.
• Issue Bug: Per-engine job concurrency blocks workflow_dispatch issue workflows from running in parallel #18101 addressed per-engine concurrency blocking parallel runs.
• The concurrency.job-discriminator feature already solves part of this (fan-out without cancellation), but doesn't provide matrix parameters or parallel agent instances within a single workflow run.

Alternatives Considered

Approach	Limitation
workflow_dispatch fan-out (dispatch N separate runs)	No shared context, hard to aggregate, per-engine concurrency blocks
job-discriminator with dispatch-ops	No matrix parameters in the agent prompt
Custom job with matrix + assign-to-agent safe output	Fragile, adds a full extra job layer, not first-class