Bump Go toolchain to 1.26.1

[jlewi]

What

Bump the Runme Go toolchain baseline to 1.26.1:

• set the module directive in go.mod to go 1.26.1
• run CI lint/build/parser jobs on Go 1.26.1
• make test-docker use GOTOOLCHAIN=auto
• install checklocks with GOTOOLCHAIN=go1.26.1

Why

The follow-up Codex notebook work pins github.com/redpanda-data/protoc-gen-go-mcp at v0.0.0-20260323112146-887f275039cd, and that module declares go 1.26.1 in its own go.mod.

When Runme CI stays on Go 1.25, that dependency causes build/test failures from toolchain version mismatches. This branch moves main to the required Go baseline first so the Codex notebook PR can stay focused on notebook/MCP behavior instead of carrying a repository-wide toolchain bump.

The Makefile updates keep local/CI helper commands aligned with that baseline:

• checklocks@go otherwise resolves with an older Go toolchain and then fails when vetting packages that require Go 1.26
• the Docker test image currently has Go 1.25, so GOTOOLCHAIN=auto lets go test fetch the module-required toolchain

Validation

• git diff --check
• go test ./pkg/agent/...