chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/golang.org/x/crypto/ssh/agent/keyring.go

@@ -112,7 +112,7 @@ func (r *keyring) Unlock(passphrase []byte) error {
 }
 
 // expireKeysLocked removes expired keys from the keyring. If a key was added
-// with a lifetimesecs contraint and seconds >= lifetimesecs seconds have
+// with a lifetimesecs constraint and seconds >= lifetimesecs seconds have
 // elapsed, it is removed. The caller *must* be holding the keyring mutex.
 func (r *keyring) expireKeysLocked() {
 	for _, k := range r.keys {
@@ -175,6 +175,15 @@ func (r *keyring) Add(key AddedKey) error {
 		p.expire = &t
 	}
 
+	// If we already have a Signer with the same public key, replace it with the
+	// new one.
+	for idx, k := range r.keys {
+		if bytes.Equal(k.signer.PublicKey().Marshal(), p.signer.PublicKey().Marshal()) {
+			r.keys[idx] = p
+			return nil
+		}
+	}
+
 	r.keys = append(r.keys, p)
 
 	return nil