Introduce upgrade and remove command

[jaycangel]

Current issue:
There doesn’t seem to be an easy way to upgrade or completely remove spec-kit once it’s been added to an existing project. Because spec-kit installs files across many folders, cleaning it up or replacing it is cumbersome and error-prone.

Suggested improvement:
Provide a built-in command or documented process to upgrade or uninstall spec-kit from a project. This could include:

• A CLI command like spec-kit upgrade or spec-kit remove.
• A manifest or tracking mechanism that records all installed files, making cleanup straightforward.
• Clear documentation on safe manual removal steps if an automated option isn’t feasible.