feat(init): record init URL

[git-hulk]

Summary

• persist an explicitly supplied init URL as init_url in ~/.clime/plugins.yaml
• reuse the recorded URL when clime init runs without an argument
• replace the recorded URL after a newly supplied URL is fetched successfully
• keep local manifest paths as one-off overrides that do not replace the recorded URL
• update CLI help, README examples, and tests

Why

Users currently need to repeat their organization’s manifest URL on every clime init run. Recording the last successful URL makes subsequent initialization consistent while retaining the built-in defaults for users who have never supplied one.

Validation

• go test ./...
• go vet ./...
• go build ./...
• git diff --check