chore: upgrade Go from 1.25.7 to 1.26.5

Go 1.26.5 released Jul 7, 2026. Includes security fix for GO-2026-5856 (crypto/tls ECH leak). Updates: CI runner (4x setup-go), go.mod, README, obsidian docs.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

README.md

@@ -1,6 +1,6 @@
 # Crussell
 
-Nail salon booking platform — Go 1.25 backend + SvelteKit 5 SPA + PostgreSQL 17 + Docker. Built for a UK sole-trader nail artist. UK-only (Cloudflare geo-block), UK phone format. All timestamps UTC-normalised — the backend's `clock.Now()` returns UTC, the DB connection uses `timezone = "UTC"`, and the frontend converts between UTC and wall-clock time client-side. Single-employee business.
+Nail salon booking platform — Go 1.26.5 backend + SvelteKit 5 SPA + PostgreSQL 17 + Docker. Built for a UK sole-trader nail artist. UK-only (Cloudflare geo-block), UK phone format. All timestamps UTC-normalised — the backend's `clock.Now()` returns UTC, the DB connection uses `timezone = "UTC"`, and the frontend converts between UTC and wall-clock time client-side. Single-employee business.
 
 ## Features