Fix post-steps indentation errors in YAML workflow generation

[Copilot]

Problem

The generatePostSteps function was generating post-steps with incorrect indentation, causing YAML schema validation failures when compiling agentic workflows. Post-steps were rendered with 4-space indentation instead of the required 6-space indentation for GitHub Actions workflow steps.

This resulted in validation errors like:

workflow schema validation failed: failed to parse YAML for schema validation: 
[1249:5] non-map value is specified
> 1249 |     - name: Post AI Step
             ^

Solution

Updated the generatePostSteps function in pkg/workflow/compiler.go to generate correct indentation that matches GitHub Actions schema requirements:

• List items (- name:): 6 spaces instead of 4
• Properties (run:, uses:, with:): 8 spaces instead of 4
• Nested properties (under with:): 10 spaces instead of 6

Before (Incorrect)

jobs:
  agent:
    steps:
      - name: AI Execution
        run: ./run-ai
    - name: Post AI Step              # ❌ 4 spaces (should be 6)
    run: echo "test"                  # ❌ 4 spaces (should be 8)

After (Correct)

jobs:
  agent:
    steps:
      - name: AI Execution
        run: ./run-ai
      - name: Post AI Step            # ✅ 6 spaces
        run: echo "test"              # ✅ 8 spaces

Additional Improvements

• Added trailing whitespace trimming to prevent stray spaces in generated YAML
• Added explicit empty line handling to avoid generating blank lines with whitespace
• Improved code comments to document the expected indentation levels

Testing

• Added comprehensive test TestPostStepsIndentationFix that validates indentation at all nesting levels
• Added test agentic workflow .github/workflows/test-post-steps.md to catch future compile issues with post-steps
• All existing post-steps tests continue to pass
• Successfully compiled all 65 workflow files in the repository with make recompile
• Manual verification confirms workflows now validate successfully

Fixes #1867

Original prompt

---

This section details on the original issue you should resolve

<issue_title>post-steps intendation errors</issue_title>
<issue_description>locally copilot made these to workaround:

• Here's the tiny patch I made to the githubnext/gh-aw CLI so post-steps render with valid YAML indentation:

  diff --git a/pkg/workflow/compiler.go b/pkg/workflow/compiler.go
  index d212984..2d4f67d 100644
  --- a/pkg/workflow/compiler.go
  +++ b/pkg/workflow/compiler.go
  @@ -2936,11 +2936,15 @@ func (c *Compiler) generatePostSteps(yaml *strings.Builder, data *WorkflowData)
           lines := strings.Split(data.PostSteps, "\n")
           if len(lines) > 1 {
               for _, line := range lines[1:] {
  -                // Remove 2 existing spaces, add 6
  -                if strings.HasPrefix(line, "  ") {
  -                    yaml.WriteString("    " + line[2:] + "\n")
  -                } else {
  -                    yaml.WriteString("    " + line + "\n")
  -                }
  +                trimmed := strings.TrimRight(line, " ")
  +                if strings.TrimSpace(trimmed) == "" {
  +                    yaml.WriteString("\n")
  +                    continue
  +                }
  +                if strings.HasPrefix(line, "  ") {
  +                    yaml.WriteString("        " + line[2:] + "\n")
  +                } else {
  +                    yaml.WriteString("      " + line + "\n")
  +                }
               }
           }

  What changed and why

  - The generator previously prepended only four spaces, which produced YAML like     - name: … under jobs.agent.steps. GitHub's workflow schema expects list items there
    to start at six spaces (      - …), so schema validation failed whenever we added a post-steps block.
  - I also trim right-hand whitespace and skip empty lines explicitly so the compiler doesn't spew stray spaces when authors leave blank lines inside post-steps.

  After rebuilding (go1.24.5 build ./cmd/gh-aw) and copying the new binary to ~/.local/share/gh/extensions/gh-aw/gh-aw, gh aw compile emits correctly indented YAML, and
  all five workflows now compile without errors.
```</issue_description>

<agent_instructions>Fix the issue reported .</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

Fixes #1867

---

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[github-actions]

Agentic Changeset Generator triggered by this pull request.

[pelikhan]

Add 1 test agentic workflow with post jobs to catch compile issues.

[pelikhan]

@copilot Add 1 test agentic workflow with post jobs to catch compile issues.

[Copilot]

@copilot Add 1 test agentic workflow with post jobs to catch compile issues.

Added test-post-steps.md workflow in .github/workflows/ that demonstrates post-steps compilation with proper indentation validation. The workflow includes multiple post-steps with various configurations (conditional execution, artifact upload, step summary) and compiles successfully. See commit for details.