feat(labeler): Extend auto-labeler to set Projects, Milestones, and Relationships

[Snider]

Summary

The auto-labeler action currently only adds labels to issues. Extend it to also configure:

• Projects: Auto-assign issues to relevant GitHub Projects
• Milestones: Set milestone based on issue type/labels
• Relationships: Set parent/child relationships, linked issues

Context

Triggered by observing the labeler working on Snider/core-1#1 - it added the enhancement label automatically but Projects/Milestones were left empty.

Proposed Behaviour

# Example config extension
labeler:
  rules:
    - match:
        title: "^feat"
        labels: [enhancement]
      actions:
        add_labels: [enhancement]
        set_project: "Core CLI"
        set_milestone: "v0.2.0"
        
    - match:
        labels: [bug, critical]
      actions:
        set_project: "Bug Triage"
        set_milestone: "Next Patch"

Acceptance Criteria

• Support set_project action in labeler config
• Support set_milestone action in labeler config
• Support add_relationship action (parent/child/related)
• Document new config options
• Backwards compatible with existing label-only configs

Related

• Observed on: feat(test): Add smart test detection to core test Snider/core-1#1
• Workflow issue: host-uk/core-agent#122