Add `skillfile info <name>` command for detailed single-entry view

[eljulians]

When you have 15+ entries, sometimes you just want to quickly inspect one without reading the Skillfile, cross-referencing the lock file, and checking the patches dir.

$ skillfile info browser
  Name:       browser
  Type:       skill
  Source:     github (anthropics/skills)
  Path:       skills/browser.md
  Ref:        main
  Locked:     sha abc1234def (2026-03-15)
  Pinned:     yes (.skillfile/patches/skills/browser.patch)
  Modified:   no
  Installed:  ~/.claude/skills/browser/SKILL.md
              .cursor/skills/browser/SKILL.md
  Cache:      .skillfile/cache/skills/browser/SKILL.md

Everything about one entry in one place: source, lock state, patch state, installed paths across all targets, cache state.

No network calls, just reads manifest, lock, patch dir, and checks installed paths on disk. Implementation in cli/src/commands/info.rs.