Add support for SSH-signed commits

[basnijholt]

This adds support for verifying commits signed with SSH keys, alongside the existing GPG support.

I use SSH keys to sign my commits, and GitHub has supported SSH commit verification since August 23, 2022. This makes SSH signing a practical alternative to GPG for people who already manage SSH keys as part of their normal Git workflow.

The main new option is:

services.comin.sshAllowedSignersPath = "/path/to/allowed_signers";

The file uses OpenSSH’s allowed signers format, so a simple entry looks like:

user@example.com ssh-ed25519 AAAA...

A commit is accepted if it verifies against either a configured GPG public key or a configured SSH allowed signer.

I kept the SSH parser intentionally conservative. It supports plain public-key entries and optional namespaces="git" restrictions, but rejects unsupported allowed-signers features instead of guessing. That includes unsupported options, ambiguous namespace patterns, SSH cert keys, and malformed entries.

I also updated the docs and regenerated the module options.

Tested with:

go build -v ./...
go test -race -v ./...
nix flake check

I also added live SSH-signing tests that generate an SSH key, create a real git commit -S commit, and verify it through comin’s update path.

[nlewo]

Thank you for this contribution which has been already asked by several users!

[basnijholt]

I had the branch reviewed by Claude (Fable 5), which caught a few more things, fixed in 31b3f47:

• SHA-1 signatures were accepted for RSA keys. x/crypto's Verify allows the legacy ssh-rsa (SHA-1) format, which OpenSSH's sshsig refuses. Now rejected (along with ssh-dss), with a test.
• Error shadowing. With both GPG and SSH configured, only the SSH failure was reported; both errors are now joined.
• Redundant parsing. The allowed signers file was re-parsed on every update; it's now parsed once at startup.

[basnijholt]

Fixed the lint in ded5429. Happy to address any other concerns!

[nlewo]

To be honest, I'm not feeling really confident by merging this PR since it's hard to be sure we will not miss any attack screnario...

I noticed the go-git project is implementing the support of SSH signed commit in go-git/go-git#1869 and go-git/x#13. I would be really nice to use this codebase instead of having to maintain our own...

I also noticed flux implemented a SSH signed commit support in fluxcd/pkg#1141.
Did you take a look at this PR? Maybe this could be helpful to avoid some mistakes.

I still need to take some time reviewing this PR and should be able to in the next couple of days.

[nlewo]

So, I didn't see any issue in this PR.
Moreover, go-git will bring soon the support for this feature: we could then remove all this code from comin when supported by go-git.

@basnijholt Thank you for your contribution!

[nlewo]

Deployed on my infra and no regression observed.

[basnijholt]

@nlewo thanks for merging 🎉 😄