fix: use SSH agent when identity files don't exist

[rfay]

Fixes "ssh: no key found" errors when using SSH agent without physical key files on disk (e.g., DDEV tests with ddev-ssh-agent).

For example:

[WARN]  2026/01/28 09:54:55 ssh: no key found &{0x4000084120}
[WARN]  2026/01/28 09:54:55 ssh: no key found &{0x4000084120}

This also improves the debug/warning when no key is found by showing the path instead of just a pointer.

Changes:

1. utils/shell.go:

   • Gracefully handle missing identity files, skip them and use agent keys
   • Skip ~/.ssh directory scan when SSH agent already has keys
   • Only scan directory as last resort to avoid warnings for non-key files

2. utils/sshportal.go:

   • Bypass machinery's sshtoken.RetrieveToken entirely when using agent-only
   • Implement retrieveTokenViaAgent() to directly use SSH agent keys
   • Routes based on availability: agent-only, file-based, or error

Root cause: machinery v0.0.31's sshtoken.RetrieveToken returns errors when identity files don't exist (lines 36-40, 52-56) instead of falling back to SSH agent keys. This is a workaround until machinery is fixed upstream.

See:

• https://github.com/uselagoon/machinery/blob/main/utils/sshtoken/sshtoken.go
• Poor SSH key handling: overly coupled to id_rsa private keys lagoon-cli#442

[bomoko]

Hi @rfay - thanks for this work. With the new machinery library update, I take it we can skip the bulk of this.
Is there anything from this we want to keep?

Gonna close this on the assumption that there isn't (and the messages on the other PRs in machinery) - let's open again if there is.

[rfay]

I didn't look again, but agree that machinery should have solved it, thanks!