Sshpiper support

[gi-yt]

Adds SSHPiper as the public SSH ingress path for Atlas VMs.

refer to spec/23-sshpiper.md for more details

breaking changes:

• git submodule has been added for go-plugin. you need to initialize it in order to build the go plugin inside the buildvm

[gi-yt]

@adityahase scp and sftp works. codeserver is a bit iffy (but from what i understood, thats because of the dev setup lag, not sshpiperd itself)

[greptile-apps]

Confidence Score: 4/5

The core implementation is solid, but two unresolved merge conflict markers corrupt spec documents and should be cleaned before merge.

The SSH transport, gateway configuration, and lookup authentication logic are well-structured and tested. Two raw conflict markers left in spec files break the documents and are the primary blocker.

spec/03-bootstrapping.md and spec/README.md contain unresolved merge conflict markers; atlas/atlas/sshpiper.py has the URL-parameter token issue.

_{Reviews (2): Last reviewed commit: "Update spec/03-bootstrapping.md" | Re-trigger Greptile}