chore: declare go 1.26 rather than 1.26.4 across the workspace

The 1.26.4 floor meant a base toolchain of 1.26.2 could not load the
workspace at all: the terminal worked only because GOTOOLCHAIN=auto
silently swapped in a downloaded 1.26.4, and gopls -- which does not get
that switch -- failed every packages.Load with

  go: go.work requires go >= 1.26.4 (running go 1.26.2)

Nothing needed the patch-level floor. agent/go.mod already declared plain
go 1.26, so this makes the workspace uniform rather than introducing a new
convention, and CI is unaffected because golang:1.26 is already newer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

shared/go.mod

@@ -1,6 +1,6 @@
 module github.com/mrhid6/vantage/shared
 
-go 1.26.4
+go 1.26
 
 require (
 	github.com/google/uuid v1.6.0