Implement robust auto-update system for GuruConnect agent

Features:
- Agent checks for updates periodically (hourly) during idle
- Admin can trigger immediate updates via dashboard "Update Agent" button
- Silent updates with in-place binary replacement (no reboot required)
- SHA-256 checksum verification before installation
- Semantic version comparison

Server changes:
- New releases table for tracking available versions
- GET /api/version endpoint for agent polling (unauthenticated)
- POST /api/machines/:id/update endpoint for admin push updates
- Release management API (/api/releases CRUD)
- Track agent_version in machine status

Agent changes:
- New update.rs module with download/verify/install/restart logic
- Handle ADMIN_UPDATE WebSocket command for push updates
- --post-update flag for cleanup after successful update
- Periodic update check in idle loop (persistent agents only)
- agent_version included in AgentStatus messages

Dashboard changes:
- Version display in machine detail panel
- "Update Agent" button for each connected machine

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

proto/guruconnect.proto

@@ -276,6 +276,7 @@ message AgentStatus {
     int64 uptime_secs = 4;
     int32 display_count = 5;
     bool is_streaming = 6;
+    string agent_version = 7;           // Agent version (e.g., "0.1.0-abc123")
 }
 
 // Server commands agent to uninstall itself
@@ -287,7 +288,38 @@ message AdminCommand {
 enum AdminCommandType {
     ADMIN_UNINSTALL = 0;            // Uninstall agent and remove from startup
     ADMIN_RESTART = 1;              // Restart the agent process
-    ADMIN_UPDATE = 2;               // Download and install update (future)
+    ADMIN_UPDATE = 2;               // Download and install update
+}
+
+// ============================================================================
+// Auto-Update Messages
+// ============================================================================
+
+// Update command details (sent with AdminCommand or standalone)
+message UpdateInfo {
+    string version = 1;             // Target version (e.g., "0.2.0")
+    string download_url = 2;        // HTTPS URL to download new binary
+    string checksum_sha256 = 3;     // SHA-256 hash for verification
+    bool mandatory = 4;             // If true, agent must update immediately
+}
+
+// Update status report (agent -> server)
+message UpdateStatus {
+    string current_version = 1;     // Current running version
+    UpdateState state = 2;          // Current update state
+    string error_message = 3;       // Error details if state is FAILED
+    int32 progress_percent = 4;     // Download progress (0-100)
+}
+
+enum UpdateState {
+    UPDATE_IDLE = 0;                // No update in progress
+    UPDATE_CHECKING = 1;            // Checking for updates
+    UPDATE_DOWNLOADING = 2;         // Downloading new binary
+    UPDATE_VERIFYING = 3;           // Verifying checksum
+    UPDATE_INSTALLING = 4;          // Installing (rename/copy)
+    UPDATE_RESTARTING = 5;          // About to restart
+    UPDATE_COMPLETE = 6;            // Update successful (after restart)
+    UPDATE_FAILED = 7;              // Update failed
 }
 
 // ============================================================================
@@ -335,5 +367,9 @@ message Message {
 
         // Admin commands (server -> agent)
         AdminCommand admin_command = 70;
+
+        // Auto-update messages
+        UpdateInfo update_info = 75;        // Server -> Agent: update available
+        UpdateStatus update_status = 76;    // Agent -> Server: update progress
     }
 }