refactor: use skillimage packages for skill.yaml parsing and discovery

[pavelanni]

Summary

Replace docsclaw's local skill-related packages with imports from
skillimage to avoid
duplication and keep the two projects aligned.

What to replace

docsclaw package	skillimage replacement	Notes
pkg/skills/card/	skillimage/pkg/skillcard	SkillCard parsing, validation, serialization
pkg/skills.ParseSkillYAML()	skillimage/pkg/skillcard.Parse()	Loose skill.yaml parse in a2a.go
pkg/skills.SkillYAML struct	skillimage/pkg/skillcard.SkillCard	Metadata struct

What to keep in docsclaw

• pkg/skills/loader.go — Discover(), LoadContent(),
  BuildSummary() (runtime skill loading, agent-specific)
• pkg/skills/a2a.go — ToAgentSkills(), MergeSkills()
  (AgentCard population, A2A-specific)

These functions would call into skillimage types instead of local
ones.

Skillimage packages available

Package	Purpose
pkg/skillcard	Parse, validate, serialize skill.yaml (any apiVersion)
pkg/source	Discover skills from directories or git repos
pkg/installed	Scan agent directories for installed skills
pkg/collection	Parse SkillCollection YAML, generate K8s manifests
pkg/lifecycle	Skill status transitions (draft → testing → published)

Steps

1. Add github.com/redhat-et/skillimage as a dependency
2. Replace pkg/skills/card/ types with skillcard.SkillCard
3. Replace ParseSkillYAML() with skillcard.Parse()
4. Update ToAgentSkills() to map skillcard.SkillCard → a2a.AgentSkill
5. Remove pkg/skills/card/ package
6. Verify skill list and Discover() still work

Context

Both projects are maintained by the same team. The skillimage
project owns the skill.yaml schema (currently
skillimage.io/v1alpha1) and should be the single source of truth
for parsing and validation. Docsclaw currently has its own
docsclaw.io/v1alpha1 schema in pkg/skills/card/ plus a loose
SkillYAML struct — both should be replaced by the skillimage
types.