perf(formatters): hoist SCP git URL regex to package level in IsValidGitURL

[matiasinsaurralde]

Summary

• Move the SCP-style git URL regex (user@host:repo) from inside IsValidGitURL to a package-level var, so it is compiled once at init instead of on every call.
• No behavior change for HTTP(S), ssh://, or SCP URL validation.

Benchmark

BenchmarkIsValidGitURL on Apple M4 Pro (darwin/arm64), 10 runs each:

URL type	Before	After	Change
HTTPS	~113 ns, 1 alloc	~112 ns, 1 alloc	unchanged (expected)
SSH-scheme	~150 ns, 2 allocs	~149 ns, 2 allocs	unchanged (expected)
SCP	~2.6 µs, 5.6 KB, 75 allocs	~243 ns, 0 B, 0 allocs	~11× faster, zero heap allocs

The win is entirely on the SCP path (git@github.com:user/repo), which previously called regexp.MustCompile on every invocation.

[fproulx-boostsecurity]

Thanks @matiasinsaurralde LGTM