[API] Add HATEOAS Links for Discoverability

[fentz26]

Problem

No hypermedia links in responses:

• Clients must hardcode URLs
• API not self-documenting
• Navigation requires out-of-band knowledge

Proposed Solution

Link Headers and Body Links

type TaskResponse struct {
    Task
    Links map[string]Link `json:"_links"`
}

type Link struct {
    Href   string `json:"href"`
    Method string `json:"method,omitempty"`
    Title  string `json:"title,omitempty"`
}

// Example response
{
    "id": "task-123",
    "title": "Implement feature",
    "status": "pending",
    "_links": {
        "self": {"href": "/v1/tasks/task-123"},
        "claim": {"href": "/v1/tasks/task-123/claim", "method": "POST"},
        "runs": {"href": "/v1/tasks/task-123/runs"},
        "parent": {"href": "/v1/tasks"}
    }
}

Acceptance Criteria

• _links object in all responses
• self link always present
• Related resource links
• Action links with methods
• Link header for pagination

References

• Parent Epic: 🔌 Epic: API Improvements #72