Validate --repository URLs and name the flag in the error (#123)

[rochala]

Fixes #123.

Scope

The issue names two repository sources: the -r/--repository flag and maven.repositories in cellar.conf. The config source comes from #121, which is still open — there is no config-file repository path on main yet. This PR fixes the pre-existing gap the issue calls out: -r was never validated.

RepositoryUrl.parse returns the rejection reason without naming a source, so the caller supplies it. When #121 lands, its config path can reuse the same check and prefix reasons with the config file instead of the flag.

What changed

• cli/src/cellar/cli/RepositoryUrl.scala (new) — parse(raw): Either[String, MavenRepository]. Trims, then rejects empty strings, unparseable URIs, a missing scheme, schemes other than http/https/file, and http(s) URLs with no host.
• cli/src/cellar/cli/CellarApp.scala — extraReposOpt uses mapValidated, prefixing each reason with Invalid --repository:. Dropped the now-unused MavenRepository import.
• cli/test/src/cellar/cli/RepositoryUrlTest.scala (new) — 7 tests over accepted and rejected forms.
• README.md — the -r flag row states the accepted schemes.

Verification

Rejected before, the bad URL and the flag both named:

$ cellar get-external -r artifactory.company.com/maven org.typelevel:cats-core_3:2.10.0 cats.Monad
Invalid --repository: 'artifactory.company.com/maven' has no scheme (expected one of http://, https://, file://)
Usage: cellar get-external [...]

Exits non-zero instead of surfacing later as CoordinateNotFound with coordinate suggestions. A valid -r https://repo1.maven.org/maven2 still renders cats.Monad correctly end-to-end.

./mill __.test (623 tasks) and ./mill _.fix --check both pass.

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