new-exec-type

Add a new executable type to the flow runner for: $ARGUMENTS

An "executable type" is a new automation block users can declare in .flow files (like exec, serial, parallel, render). Follow these steps in order:

1. Schema first — add the new type's fields to types/executable/executable_schema.yaml. Read the existing schema to match the structure. Regenerate with the generate executable (mcp__flow__execute, ref generate) — it rewrites types/executable/executable.gen.go. Never edit the .gen.go file directly.

2. Runner handler — each type is its own package: create internal/runner/<type>/<type>.go. Read internal/runner/exec/exec.go or internal/runner/serial/serial.go as reference for the exact interface and pattern.

3. Register the type — wire the new handler into internal/runner/runner.go (the dispatch table).

4. Parser support — update internal/fileparser/ if needed to recognize and validate the new type during YAML parsing.

5. Tests — add unit tests in internal/runner/<type>/<type>_test.go using Ginkgo. Use Describe/It/Entry — never FDescribe/FIt. Cover happy path and error cases.

6. Validate — run the validate executable (mcp__flow__execute, ref validate) to confirm generate, lint, and tests all pass.

Do not skip the schema step — editing generated files directly will cause CI to fail.