build: require Go 1.26 everywhere at once

The newest go-json-experiment/json, which wails/v3's application
package imports, declares go 1.26, so taking it raises our go
directive with it.  Every other place that names a Go version moves in
the same commit: GO_VERSION in ci, desktop-assets and android-apk, the
Arch makedepends, and CONTRIBUTING's table.

index-artifact's container image moves too, and it is the one that
matters most.  The official golang images set GOTOOLCHAIN=local, so a
golang:1.25 container refuses a go 1.26 module outright rather than
fetching a newer toolchain, and that job owns the ~205 GB checkpoint.

Closes #265

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

go.mod

@@ -1,6 +1,6 @@
 module yellowjacket
 
-go 1.25.0
+go 1.26
 
 require (
 	github.com/BurntSushi/toml v1.6.0
@@ -144,7 +144,7 @@ require (
 	github.com/go-git/gcfg v1.5.1-0.20230307220236-3a3c6141e376 // indirect
 	github.com/go-git/go-billy/v5 v5.9.0 // indirect
 	github.com/go-git/go-git/v5 v5.19.2 // indirect
-	github.com/go-json-experiment/json v0.0.0-20251027170946-4849db3c2f7e // indirect
+	github.com/go-json-experiment/json v0.0.0-20260820222146-c27c302e5fc3 // indirect
 	github.com/go-ole/go-ole v1.3.0 // indirect
 	github.com/go-resty/resty/v2 v2.17.1 // indirect
 	github.com/go-sql-driver/mysql v1.9.3 // indirect