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>

agent/src/main.rs

@@ -23,6 +23,7 @@ mod session;
 mod startup;
 mod transport;
 mod tray;
+mod update;
 mod viewer;
 
 pub mod proto {
@@ -118,6 +119,10 @@ struct Cli {
     /// Enable verbose logging
     #[arg(short, long, global = true)]
     verbose: bool,
+
+    /// Internal flag: set after auto-update to trigger cleanup
+    #[arg(long, hide = true)]
+    post_update: bool,
 }
 
 #[derive(Subcommand)]
@@ -182,6 +187,12 @@ fn main() -> Result<()> {
     info!("GuruConnect {} ({})", build_info::short_version(), build_info::BUILD_TARGET);
     info!("Built: {} | Commit: {}", build_info::BUILD_TIMESTAMP, build_info::GIT_COMMIT_DATE);
 
+    // Handle post-update cleanup
+    if cli.post_update {
+        info!("Post-update mode: cleaning up old executable");
+        update::cleanup_post_update();
+    }
+
     match cli.command {
         Some(Commands::Agent { code }) => {
             run_agent_mode(code)