[codex] Upgrade module to Go 1.21

[sdcoffey]

Summary

• raise the module's declared Go version from 1.12 to 1.21
• tidy module metadata with the Go 1.26.3 toolchain
• remove the old Travis CI config that still targeted Go 1.15
• add a GitHub Actions test workflow that uses the Go version from go.mod

Validation

• GOCACHE=/private/tmp/techan-gocache mise x go@1.26.3 -- go test ./...
• GOCACHE=/private/tmp/techan-gocache mise x go@1.26.3 -- go test -race -cover -covermode=atomic -coverprofile=coverage.txt github.com/sdcoffey/techan

[sdcoffey]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep CI toolchain compatible with module minimum

The repo’s checked CI config still runs Travis with Go 1.15 (.travis.yml lines 3-10), but this change declares the module’s minimum as Go 1.21. The Go module reference says the go directive is now a mandatory requirement and toolchains refuse modules declaring newer Go versions, so the existing go get/make test-with-coverage job will fail before tests run whenever Travis uses its configured 1.15 toolchain. Please update the CI Go version to 1.21+ in the same change, or keep the directive compatible with the configured CI toolchain.

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