fix(ci,nix): resolve nix-update failures

- Add mkdir -p before lock file usage in nix-update workflow
- Patch beads go.mod to accept go 1.25.5 instead of 1.25.6

Fixes two independent issues blocking nix-update CI:
1. Missing lock file directory causing flock to fail
2. beads v0.49.3 requires go >= 1.25.6, environment has go 1.25.5

Both fixes verified with grep and diagnostics.

.gitea/workflows/nix-update.yml

@@ -330,6 +330,7 @@ jobs:
             fi
           fi
 
+          mkdir -p "/tmp/$USER"
           LOCKFILE="/tmp/$USER/nix-update-git-lock"
 
           trap 'rm -f "$LOCKFILE"; exit' EXIT INT TERM