Add `posit connect skills install` command

Summary

Add a posit connect skills install command that installs one or more Posit Connect agent skills into a supported agent skill directory.

Proposed behavior

• Support installing skills into either the global/user skill directory or the current project skill directory.
• Allow one or more skills to be selected explicitly from the command line.
• When multiple Connect skills are available and none are specified, either:
  • install all available skills, or
  • provide an interactive questionnaire that lets the user select which skills to install.
• Clearly report which skills were installed and where they were written.
• Handle an existing installation predictably (for example, prompt, require a force flag, or skip with a clear message).

Example interface

posit connect skills install <skill>... [--global | --project]
posit connect skills install --all [--global | --project]
posit connect skills install [--global | --project]  # interactive selection when supported

The exact flags and default selection behavior can follow existing CLI conventions.

Acceptance criteria

• Users can install at least one Connect agent skill with a single command.
• Users can install multiple skills in one invocation.
• Users can target both global/user and project-local skill directories.
• Multiple available skills can be installed all at once or selected through an interactive flow.
• Existing destination files are handled without silent destructive overwrites.
• Command help documents selection behavior, destination scope, and overwrite behavior.
• Automated tests cover global and project installs, multiple skills, and existing-install behavior.

No files, entry points, or tests are named. Start by locating the existing CLI conventions and agent-skill installation code, then define the command's selection, destination, and overwrite behavior. Done means global and project installs, single and multiple skills, safe existing-file handling, documented help, and automated coverage for the listed cases.