Add local release uninstall command

[piraz]

Overview

Add a command to remove an installed Neovim release from the local NvimIM
installation path.

Proposed command

nvimim uninstall <version>

Requirements

• Require exactly one release version argument.
• Remove only releases installed locally by NvimIM.
• Never download, install, or query GitHub.
• Fail clearly when the requested release is not installed.
• Refuse to uninstall the release currently targeted by the current symlink.
• Tell the user to select another installed release before uninstalling the
  active one.
• Preserve the current symlink, configuration files, and release metadata
  cache.
• Report which release was removed.

Safety

• Do not remove arbitrary paths outside the configured installation directory.
• Validate that the target is an installed release directory before removal.
• Add confirmation support consistent with install and upgrade behavior, with a
  --yes option for non-interactive use.

Acceptance criteria

• nvimim uninstall <version> removes an installed inactive release.
• The command rejects missing releases.
• The command rejects the active release.
• The command never contacts GitHub.
• Unit tests cover successful removal and safety failures.
• README documents the command and its active-release restriction.