Add a `--skip-install` flag to `fedify init`

[2chanhaeng]

Summary

Add a --skip-install flag to fedify init that skips automatic dependency installation after scaffolding.

Motivation

fedify init currently runs the package manager's install command automatically after generating a project. This is convenient in most cases, but can be undesirable when:

• Working in a CI environment where installation is handled separately.
• Using a monorepo where dependencies are installed from the root.
• The user prefers to inspect the generated files before installing.

Proposed behavior

• Default (no flag): install dependencies as soon as the files are ready.
• --skip-install: skip the install step entirely.

No interactive prompt is needed since installing is the sensible default.