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/connection.go

@@ -74,6 +74,13 @@ type Conn interface {
 	//   Disconnect
 }
 
+// AlgorithmsConnMetadata is a ConnMetadata that can return the algorithms
+// negotiated between client and server.
+type AlgorithmsConnMetadata interface {
+	ConnMetadata
+	Algorithms() NegotiatedAlgorithms
+}
+
 // DiscardRequests consumes and rejects all requests from the
 // passed-in channel.
 func DiscardRequests(in <-chan *Request) {
@@ -106,6 +113,7 @@ type sshConn struct {
 	sessionID     []byte
 	clientVersion []byte
 	serverVersion []byte
+	algorithms    NegotiatedAlgorithms
 }
 
 func dup(src []byte) []byte {
@@ -141,3 +149,7 @@ func (c *sshConn) ClientVersion() []byte {
 func (c *sshConn) ServerVersion() []byte {
 	return dup(c.serverVersion)
 }
+
+func (c *sshConn) Algorithms() NegotiatedAlgorithms {
+	return c.algorithms
+}