Accept status_changed in on.issues.types #77162

GitHub Actions supports on: issues: types: [status_changed], which runs when the status of an issue changes (github/rest-api-description#7448). gh-aw rejects it. Issue statuses are in preview and only available to accounts enrolled in the preview.

Implementation plan

1. pkg/parser/schemas/main_workflow_schema.json: add status_changed to on.issues.types.
2. Makefile, patch-github-actions-schema: add status_changed to the appended issue activity types, then update pkg/workflow/schemas/github-workflow.json. The jq selector expects issues.properties.types, but SchemaStore nests types under issues.oneOf, so the target appends nothing today. Select .properties.issues.oneOf[] | select(.properties?.types?) instead.
3. pkg/workflow/actionslanguageservice/compiler_schema.go: add status_changed to issues-activity-type in the compiler overlay, next to cache-mode. Copy the slice so the pinned upstream schema doesn't change.

Acceptance criteria

• gh aw compile accepts types: [status_changed] and still rejects status_change.
• Tests in pkg/parser/schema_test.go, pkg/workflow/schema_validation_test.go and pkg/workflow/actionslanguageservice/compiler_schema_test.go. make agent-finish passes.