chore(go): pin go directive to 1.26.1

[onedr0p]

Fleet policy change (home-operations/renovate-config#98): the go directive in go.mod is a compatibility floor, not the build toolchain, so it now pins to the lowest 1.26 release the dependencies allow instead of tracking the mise toolchain pin; Renovate no longer updates it.

For this repo the floor is 1.26.1, forced by sigs.k8s.io/external-dns; it can drop further once that dependency ships with a lower requirement.

Verified with go mod tidy (no further changes) and go build ./... on Go 1.26.5. The AGENTS.md Go bullet now describes the pinned state.

[greptile-apps]

Greptile Summary

The PR separates the module compatibility floor from the repository's build toolchain by lowering the go directive to 1.26.1.

• Pins go.mod to the lowest currently required Go 1.26 patch release.
• Updates contributor guidance to document that Renovate no longer keeps the directive aligned with the mise toolchain.

Confidence Score: 5/5

The PR appears safe to merge with no actionable changed-code defects identified.

The directive-only change does not alter dependency selection or the configured build toolchain, and the accompanying guidance accurately documents the new compatibility-floor policy.

Important Files Changed

Filename	Overview
go.mod	Lowers only the Go compatibility directive from 1.26.6 to 1.26.1 without changing the dependency graph.
AGENTS.md	Updates Go-version guidance to distinguish the compatibility floor from the independently pinned build toolchain.

_{Reviews (1): Last reviewed commit: "chore(go): pin go directive to 1.26.1" | Re-trigger Greptile}