fix: upgrade Go to 1.26.9 for standard library security fixes

[fullstackjam]

What does this PR do?

Raise the minimum Go version from 1.26.8 to 1.26.9 so CI and release builds use the patched standard library.

Why?

The govulncheck drift sensor reports nine reachable standard library vulnerabilities on Go 1.26.8, all fixed in Go 1.26.9. All build workflows already read the version from go.mod.

Fixes #173.

Testing

• go vet ./... passes
• Relevant existing tests pass: go test -race -timeout 5m ./... (no application logic changed, so no new tests)
• Tested locally: go run golang.org/x/vuln/cmd/govulncheck@latest ./... reports No vulnerabilities found.
• go mod tidy -diff and git diff --check pass

Cross-repo checklist

• No docs/content update needed in openboot.dev.
• No CLI ↔ server API contract change; openboot-contract is unaffected.

Notes for reviewer

One-line toolchain patch update; no application code or dependency versions changed. CI and release jobs use go-version-file: go.mod. Reverting this commit restores the previous toolchain but also restores the reported vulnerabilities.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-15T13:46:54.598113Z	d91a7e9	PR opened

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