refactor: rename module to github.com/timimsms/cu

[timimsms]

Summary

• Renames the Go module from github.com/tim/cu to github.com/timimsms/cu (the repository's real home), fixing go install — which cannot work while the module path differs from the repo URL.
• Updates all Go imports (27 files), plus historical references in README.md, internal/auth/mock/README.md, and docs/project-buildout.
• Updates the four ldflags -X paths in both the Makefile and .goreleaser.yml in lockstep so version stamping stays intact.
• .goreleaser.yml release footer: fixes the go install path and narrows the Homebrew heading to ### Homebrew (macOS) (casks are macOS-only).
• Adds the GoReleaser-documented post-install hook to the homebrew_casks entry to strip com.apple.quarantine from the unsigned binary.
• Runs gofmt -w across cmd and internal (normalizes 6 previously gofmt-dirty files and sed output).

Verification

• grep -rn 'github.com/tim/cu' across the repo returns nothing
• go build ./..., go vet ./..., go test -race ./... all pass
• goreleaser check validates the config
• goreleaser build --snapshot --clean --single-target succeeds; the produced binary reports cu version 0.0.1-dev with commit, date, and Built by: goreleaser — version stamping works with the new -X paths

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