Add Docker SSH end-to-end test

[Soph]

Summary

• add an opt-in Docker-backed SSH end-to-end sync test
• exercise real sshd plus git-upload-pack/git-receive-pack through syncer.Run
• add a mise task and testing docs entry for the new SSH Docker test

Testing

• env GOCACHE=/tmp/go-build go test ./internal/syncer -run 'TestRun_SSHDockerSync|TestRun_IntegrationSyncOverSSHShimV2'
• mise run test:ssh-docker

Stacked on top of #54.

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Note

Low Risk
Low risk since changes are isolated to opt-in integration testing and developer tooling, with no production code paths modified.

Overview
Adds a new opt-in end-to-end test (TestRun_SSHDockerSync) that spins up an sshd + git server in Docker, injects an ephemeral SSH key/config via a wrapper ssh, and verifies syncer.Run can bootstrap-sync from a source bare repo to a target over real git-upload-pack/git-receive-pack.

Documents how to run the Docker SSH test in docs/testing.md and adds a mise task (test:ssh-docker) to execute it.

^{Reviewed by Cursor Bugbot for commit 8607211. Configure here.}