fix: resolve SCP-like submodule URLs via transport.NewEndpoint

[mafredri]

Implements two suggestions from mafredri's review on #485 that have not been picked up yet:

• Replace url.Parse + the SCP-like guard in ResolveSubmoduleURL with go-git's transport.NewEndpoint. SCP-like parent URLs (git@host:path) now resolve correctly, including the git@host:port:path form. The documented Support SCP-like parent URLs with relative submodules #492 limitation goes away.
• Cover the new behavior with scpRelativeSibling, scpRelativeChild, scpMultiLevelUp, scpWithPort, and httpsMultiLevelUp cases. badParent and the scpParentWithRelativeSubmodule "expectErr" cases are removed because transport.NewEndpoint("://bad") parses as file:// and the SCP case no longer errors.

Threads addressed:

• feat: add git submodule support #485 (comment) (mafredri's transport.NewEndpoint patch)
• feat: add git submodule support #485 (comment) follow-up comment (scpWithPort test)

Verified locally:

• go test ./git/...: passes (all 9 TestResolveSubmoduleURL subtests pass).
• go build ./...: clean.
• golangci-lint run ./git/...: clean.

Implementation plan

1. Replace url.Parse + SCP-guard with transport.NewEndpoint in
   git/git.go::ResolveSubmoduleURL. Use parentEP.String() to
   reconstruct. Drop the limitation comment (no longer applies).
2. Update git/git_test.go::TestResolveSubmoduleURL: drop badParent
   and scpParentWithRelativeSubmodule expectErr cases; add
   scpRelativeSibling, scpRelativeChild, scpMultiLevelUp,
   scpWithPort, httpsMultiLevelUp.
3. Keep scpLikeURLRegex (still used by RedactURL and extractHost).
4. No changes to auth forwarding, SameHost, RedactURL, or
   extractHost.

Out of scope (intentional):

• AMREM-12 cross-host auth integration test. SameHost is unit-tested already. A higher-level test needs separate host identities for the two httptest.Servers and that is a real piece of work; @bjornrobertsson is already asking for guidance in-thread.
• AMREM-7 native git.Submodules().Update(). Discussed and parked due to known go-git v5 relative-path bugs.

Generated by Coder Agents on behalf of @mafredri to land mafredri's own review suggestions on #485.

🤖 This PR was created with the help of Coder Agents, and will be reviewed by a human. 🏂🏻