[plan] Add GitHub Actions minutes cost estimation to generated workflows

[github-actions]

Objective

Add GitHub Actions minutes cost estimation to generated workflows, helping users understand resource consumption before deploying scheduled automation.

Context

Research recommended cost estimation as a high-value feature, especially for scheduled workflows that run frequently (hourly, daily, weekly). This helps teams make informed decisions about automation frequency.

Approach

1. Create cost estimation logic:

   • Define average execution time per workflow type
   • Calculate minutes per run based on job count + tool usage
   • Multiply by schedule frequency (hourly = 720/month, daily = 30/month)
   • Factor in multipliers: Playwright (+5 min), repo-memory (+1 min), network API calls (+2 min)

2. Add cost table to workflow documentation:

   ## Estimated Cost
   - **Execution time**: ~3 minutes per run
   - **Schedule**: Hourly (720 runs/month)
   - **Monthly total**: ~2,160 minutes (~36 hours)
   - **Free tier impact**: 2,160 / 2,000 = 108% (exceeds free tier)
   

3. Include optimization suggestions when costs are high:

   • Reduce frequency (hourly → every 6 hours)
   • Use paths filters to skip unnecessary runs
   • Optimize tool usage (e.g., parallel jobs instead of sequential)

4. Update agent instructions to always include cost estimation for scheduled workflows

Files to Modify

• Update: .github/instructions/developer.instructions.md (add cost estimation guidelines)
• Create: .github/instructions/examples/cost-estimation-table.md (estimation reference)

Acceptance Criteria

• Cost estimation formula accounts for job count, tools, and schedule
• Cost table template includes execution time, frequency, monthly total
• Free tier impact calculated (GitHub Actions free tier = 2,000 min/month for public repos)
• Optimization suggestions provided when monthly cost exceeds 1,500 minutes
• Agent instructions updated to generate cost tables for scheduled workflows
• Tested with 3 scenarios: hourly monitoring, daily scan, weekly digest

Expected Impact

Helps users make informed decisions about automation frequency, preventing unexpected GitHub Actions minutes consumption and encouraging efficient workflow design.
Related to #10285

AI generated by Plan Command for discussion #10248

[github-actions]

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