Phase 1: Increase Argon2id to 128 MiB and add SSH agent forwarding

Argon2id:
- Increase memory parameter from 64 MiB to 128 MiB for stronger KDF

SSH Agent:
- Add agent.go with connectAgent(), agentAuthMethod(), requestAgentForwarding()
- Integrate agent auth into buildAuthMethods() (tried first when enabled)
- Set up agent forwarding after PTY allocation when agentForwarding is true
- Track and clean up agent socket connection on session close
- Add AgentAuth and AgentForwarding fields to SSHConfig model
- Add SSH Agent section to profile editor with toggle controls

internal/secrets/secrets.go

@@ -23,7 +23,7 @@ const (
 
 	// Argon2id key derivation parameters.
 	argonTime    = 3
-	argonMemory  = 64 * 1024
+	argonMemory  = 128 * 1024 // 128 MiB — deliberately strong for a desktop app
 	argonThreads = 4
 	argonKeyLen  = 32