Skills API handlers

[JAORMX]

Description

Replace the 501 Not Implemented stubs in pkg/api/v1/skills.go with basic handlers that delegate to the SkillService for storage CRUD. This is the foundation that #3651 later extends with full OCI endpoints (validate, build, push).

Parent: #3741
Dependencies: #3742
Extended by: #3651 (adds full handler logic with OCI operations, richer filtering)

Acceptance Criteria

• GET /api/v1beta/skills — list skills with optional ?scope= filter
• POST /api/v1beta/skills/install — record skill installation (metadata only — OCI pull added by REST API skills handlers #3651)
• POST /api/v1beta/skills/uninstall — uninstall skill
• GET /api/v1beta/skills/{name} — get skill info
• POST /api/v1beta/skills/validate — delegates to existing validator (already works)
• POST /api/v1beta/skills/build, POST /api/v1beta/skills/push — return appropriate "not yet implemented" until REST API skills handlers #3651
• Request body validation with appropriate error responses (400, 404, 409)
• Consistent with existing handler patterns (JSON responses, error handler wrapper)
• SPDX headers
• API tests with mock SkillService

Code Pointers

• pkg/api/v1/skills.go — existing 501 stubs and SkillsRouter
• pkg/api/v1/groups.go — reference for handler patterns
• pkg/api/v1/workloads.go — reference for query parameter handling

Size Estimate

~150 LOC code, 1 file (excluding tests)