Add opt-in auto-update from Gitea releases

The program now lives in /usr/local/lib/bamboo and is owned by its service user so it can replace itself.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

go.mod

@@ -2,7 +2,7 @@ module git.veillax.com/Veillax/bamboo
 
 go 1.27.3
 
-require git.veillax.com/Veillax/reef v0.1.0
+require git.veillax.com/Veillax/reef v0.1.3
 
 require (
 	github.com/Microsoft/go-winio v0.6.4 // indirect