fix(build): upgrade Go toolchain to 1.26.6

[plural-copilot]

Summary

• update all authoritative Go 1.26.5 pins to 1.26.6 for release builds, published images, tests, and code generation
• preserve existing build and version-selection conventions
• verify no repository-controlled 1.26.5 pins remain

Validation

• docker run ... golang:1.26.6-bookworm go test ./...
• Dockerfile static checks, YAML parsing, shell syntax checks, and exact version searches

Fixes PROD-5140

[plural-copilot]

This PR was generated by the codex Plural Agent Runtime. Here's some useful information you might want to know to evaluate the ai's perfomance:

Name	Details
💬 Prompt	## PROD-5140: remediate Go stdlib CVEs in published plural-cli artifacts...
🔗 Run history	View run history

[linear]

PROD-5140

[greptile-apps]

Greptile Summary

The PR consistently upgrades repository-controlled Go toolchain and container-image pins from 1.26.5 to 1.26.6.

• Updates the module Go version.
• Updates builder, runtime, test, and code-generation container images.
• Preserves the existing image variants and containerization conventions.

Confidence Score: 5/5

The PR appears safe to merge, with all changed toolchain pins consistently upgraded and no actionable defect identified.

The changes only replace Go 1.26.5 pins with 1.26.6 equivalents while preserving image variants, overrides, build steps, and dependency resolution.

Important Files Changed

Filename	Overview
Dockerfile	Updates both Alpine-based Go image stages from 1.26.5 to 1.26.6 without changing build or runtime behavior.
go.mod	Updates only the Go language/toolchain directive from 1.26.5 to 1.26.6; dependency versions remain unchanged.
hack/gen-client-mocks.sh	Updates the explicit mock-generation container image to Go 1.26.6.
hack/lib.sh	Updates the default containerized Go image to 1.26.6 while preserving environment overrides.
test.Dockerfile	Updates the Bookworm-based test image from Go 1.26.5 to 1.26.6.

_{Reviews (1): Last reviewed commit: "fix(build): upgrade Go toolchain to 1.26..." | Re-trigger Greptile}