Fix YAML unmarshal failure when reviewers is string in create-pull-request

[Copilot]

YAML unmarshaling failed silently when reviewers was a string instead of array, causing all create-pull-request fields (expires, title-prefix, labels, draft) to be lost from handler config.

Problem

safe-outputs:
  create-pull-request:
    expires: 2d
    title-prefix: "[TEST]"
    reviewers: copilot  # String instead of array
  push-to-pull-request-branch:

Generated handler config:

"create_pull_request": {}  // All fields missing

Root Cause
String-to-array conversion for reviewers happened after YAML unmarshaling. Parser expected []string, got string, failed silently with zero values.

Changes

• pkg/workflow/create_pull_request.go: Move reviewers string-to-array conversion before unmarshaling
• pkg/workflow/compiler_safe_outputs_pr_expires_test.go: Add regression test

Impact
Fixed 5 workflows: poem-bot, tidy, q, github-mcp-tools-report, technical-doc-writer

After fix:

"create_pull_request": {
  "expires": 48,
  "title_prefix": "[TEST]",
  "labels": ["test", "automation"],
  "draft": false
}

Original prompt

Review the problem reported in #13285 description and fix it

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[pelikhan]

@copilot push-to-pull-request-branch should NOT support expires

[Copilot]

@copilot push-to-pull-request-branch should NOT support expires

Confirmed - push-to-pull-request-branch does not and should not have expires support.

The fix only affected create-pull-request where the expires field (along with title-prefix, labels, draft) was being lost due to a YAML unmarshal failure when reviewers was specified as a string.

Current implementation:

• ✅ create-pull-request has Expires field in struct and schema
• ✅ push-to-pull-request-branch does NOT have expires field in struct or schema
• ✅ Handler config builder for push_to_pull_request_branch does NOT include expires

No code changes needed.