Skill service implementation

[JAORMX]

Description

Implement the full SkillService interface, orchestrating OCI library operations, state management, skill installation, parsing, and validation into a cohesive service layer.

Context

Part of the Skills Lifecycle Management epic. This is the central orchestration layer that ties together all skill components. Implements the SkillService interface defined in TASK-001 (#3645).

Dependencies: stacklok/toolhive-core#15, stacklok/toolhive-core#16, #3646, #3648, and Skill installer
Blocks: REST API handlers, Groups integration

Acceptance Criteria

• pkg/skills/service_impl.go implements SkillService interface
• List(): scans client skill directories, merges managed (from index) and unmanaged skills
• Install(): pulls from registry (via toolhive-core), extracts to client directory, tracks state
• Uninstall(): removes managed skill files, cleans state, refuses to remove unmanaged
• Info(): returns detailed skill information from metadata + SKILL.md frontmatter
• Validate(): runs parser + validator on skill directory, returns structured results
• Build(): uses toolhive-core packager to create OCI artifact, stores locally
• Push(): uses toolhive-core registry client to push artifact
• Upgrade support via -u flag: resolves original reference, pulls latest
• Constructor accepts all dependencies (OCI store, registry, packager, state, installer)
• Unit tests with mocked dependencies, SPDX headers

Code Pointers

• pkg/skills/service.go — interface to implement (from TASK-001)

References

• Skill types, interfaces, and command/API stubs #3645