feat(cli): `prs inspect <skill> --layers` debugging command

[mrwogu]

Context

When skills are composed across multiple registry layers (e.g., Comarch's 4-layer architecture: company → product → BU → project), debugging which layer contributed which property becomes difficult. There is no way to see the composition breakdown without manually tracing imports and extends.

Parent issue: #199 (Phase 2)
Spec: docs/superpowers/specs/2026-03-30-skill-overlay-extends-design.md §13
Review: docs/superpowers/specs/2026-03-30-skill-overlay-extends-review.md §10

Proposal

Add a new CLI command prs inspect <skill-name> --layers that shows each layer's contribution to a composed skill.

Example output

$ prs inspect clm512-expert --layers

Skill: clm512-expert
Layers: 3

Layer 1 — @clm5core/skills/clm512-expert (base)
  description: "CLM 5.12+ development expert"
  content: ✓ (247 lines)
  references: [spring-patterns.md]
  allowedTools: [Read, Bash]

Layer 2 — @bu/skills/clm512-expert (@extend)
  description: "CLM 5.12+ Expert z kontekstem BU CRM Retail" (replaces Layer 1)
  references: [architecture.md, modules.md, patterns.md, integrations.md] (appended)

Layer 3 — project/.promptscript/project.prs (@extend)
  allowedTools: [Read, Bash, Write] (replaces Layer 2)

Composed result:
  description: "CLM 5.12+ Expert z kontekstem BU CRM Retail"
  content: 247 lines
  references: 5 files (spring-patterns.md, architecture.md, modules.md, patterns.md, integrations.md)
  allowedTools: [Read, Bash, Write]

Design considerations

• Read-only command — no resolver changes needed, only inspection of the resolution pipeline
• Should work with --format json for programmatic consumption
• Must handle skill composition (@use phases) and extensions (@extend) independently
• Should show merge strategy applied per property (replace / append / shallow-merge)

Acceptance criteria

• prs inspect <skill-name> --layers shows per-layer property contributions
• Each layer shows source path and composition mechanism (@extend, @use)
• Merge strategy is indicated for overridden properties
• --format json outputs structured JSON
• Works with multi-registry configurations
• Handles missing/unresolvable skills gracefully with actionable error messages
• Unit and integration tests
• CLI help text and documentation