feature: skip mode — emit unmatched steps as skipped instead of omitting them

[JonCubed]

Problem

When a watch path doesn't match, the plugin omits the step entirely from the generated pipeline. This breaks dependency chains. If a downstream step has depends_on targeting a group or step key that was omitted, the build will fail or stall waiting for a step that was never created.

Suggestion

Add an opt-in mode where unmatched steps are emitted with skip: "No changes detected" rather than being omitted. A skipped step in Buildkite is treated as completed, so depends_on references resolve correctly.

This could be configured at the plugin level (suggested names: skip_on_no_changes or always_emit):

- monorepo-diff#v1.x.x:
    diff: "git diff --name-only HEAD~1"
    skip_on_no_changes: true
    watch:
      - path: services/
        config:
          group: Test Group
          key: group:test
          steps:
            - command: echo deploy

When skip_on_no_changes is true and the path doesn't match, the generated pipeline would include:

steps:
  - group: Test Group
    key: group:test
    skip: "No changes detected"
    steps:
      - command: echo deploy