feat(cli): add uninstall to remove the packaged skills

[ggordonhall]

Why

Removing the global package leaves installed Skills behind, and consumers need a supported reverse for dependency postinstall state.

Changes

• add an always-JSON context-tree uninstall command
• remove every directory owned by the context-tree- prefix from selected host Skill roots
• report inspected-clean hosts separately from skipped hosts
• verify the behavior through the real packed tarball and a real global install

Ownership boundary

Install and uninstall own exactly the context-tree- prefix in each host skills directory. Uninstall does not touch foreign Skills, host directories, ~/.context-tree/connections.json, managed clones under ~/.context-tree/trees/, or project AGENTS.md pointer blocks. Symlinked or non-directory layouts are reported and never followed.

Verification

• pnpm check
• pnpm typecheck
• pnpm test
• pnpm check:package