Support configurable `runs-on` for the generated agentics-maintenance workflow

[neta-vega]

Problem

The agentics-maintenance.yml workflow generated by gh aw compile hardcodes runs-on: ubuntu-slim (and ubuntu-latest for the validate_workflows job) across all jobs. There is no way to configure the runner label via the gh-aw config.

Since this file is auto-generated (by pkg/workflow/maintenance_workflow.go) and explicitly says DO NOT EDIT, any manual change to runs-on is overwritten the next time gh aw compile runs.

Impact

Organizations using self-hosted runners are forced to consume GitHub-hosted runner minutes for every scheduled and on-demand maintenance run. With the default schedule of every 2 hours, this adds up to significant unnecessary cost — especially for teams that already have idle self-hosted capacity.

Expected behavior

Allow users to configure the runs-on value for the maintenance workflow, e.g. via a top-level config field like:

maintenance:
  runs-on: [self-hosted, linux]

…or a global runs-on override that applies to all generated workflows.

Current workaround

None that survives recompilation. Manually editing the generated file is overwritten by gh aw compile.

Environment

• gh-aw version: v0.68.3
• Generated by: pkg/workflow/maintenance_workflow.go

[pelikhan]

Allow configuration in aw.json

[pelikhan]

@copilot ensure the feature is supported and documented