chore(toolchain): upgrade to Go 1.27

AGENTS.md

@@ -21,7 +21,7 @@ repository. All AI-assisted changes must:
 - **Release procedure.**
   1. Bump `version` in `justfile` and `cmd/gasm/main.go`.
   2. Update `CHANGELOG.md` with a new `## [X.Y.Z] — YYYY-MM-DD` section.
-  3. Update `README.md` and `docs/architecture.md` if user-visible
+  3. Update `README.md` and `docs/ARCHITECTURE.md` if user-visible
      behaviour changed.
   4. Run the Definition of Done (below).
   5. Commit on `development`.
@@ -55,7 +55,7 @@ No body, no footers, no trailing period on the subject.
 
 ## Code Style
 
-Language: Go 1.26 (`toolchain go1.26.5`).
+Language: Go 1.27 (`toolchain go1.27.0`).
 
 ### Formatter