[Bug]: auto-repair stale .pub instead of skipping when it doesn't pair with local .priv

[la14-1]

What happened?

Follow-up to #3395. That PR catches the silent-failure mode where ~/.ssh/id_ed25519.pub on disk doesn't pair with ~/.ssh/id_ed25519 (stale pub copied from another machine, etc.) — but only diagnoses it: the bad pair is filtered out and the user is told to fix it manually.

Real-world report from Slack: user hit SSH key 'id_ed25519' already registered with DigitalOcean followed by 33× Permission denied (publickey) on a hermes launch. With #3395 merged, they'd instead see a "skipped — does not pair" warning and then have to manually run ssh-keygen -y -P "" -f ~/.ssh/id_ed25519 > ~/.ssh/id_ed25519.pub to recover.

Proposed fix

In discoverSshKeys() (or a new helper), when verifyKeyPair() returns "mismatch":

1. Rename the stale .pub to .pub.spawn-backup-<timestamp> (never destroy user data).
2. Write the correct pub — derived from the priv via ssh-keygen -y -P "" -f <priv> — to <priv>.pub.
3. Log: Repaired ~/.ssh/id_ed25519.pub (stale public key replaced; original saved as .pub.spawn-backup-*).
4. Continue with the repaired pair — SSH handshake will now succeed because the pub registered with the cloud provider actually pairs with the local priv.

The .priv is authoritative; any .pub on disk that doesn't derive from it is wrong by definition, so the rewrite is safe. Passphrase-protected keys (verifyKeyPair === "unverifiable") continue to be skipped silently — we can't derive the pub for those without the passphrase.

Acceptance criteria

• Mismatched .pub is auto-rewritten from the matching .priv
• Original .pub is preserved at <priv>.pub.spawn-backup-<timestamp>
• Passphrase-protected keys are still skipped silently (unchanged behavior)
• Tests cover: successful repair + backup created, passphrase case still skipped, repair survives across cached discoverSshKeys() calls
• Slack user's failure reproduces as a passing e2e test (can stage a bad .pub)

References

• PR fix(ssh): auto-repair stale pub that does not pair with local priv #3395 — verification + warn-and-skip (merge first, then extend)
• Slack thread: https://openrouter.enterprise.slack.com/archives/C0AE7RTDPPH/p1778070836368039

Filed from Slack by SPA

[la14-1]

Rolled into #3395 directly — pushed commit 2a53420 onto that PR which extends verifyKeyPair from warn-and-skip to auto-repair (back up stale .pub, rewrite from derived key). No separate PR needed.