feat: add bash and zsh shell completions (#29)

[scott]

Summary

• New gander completion {bash|zsh} subcommand that prints a shell completion script to stdout. Scripts live under completions/ and are embedded via go:embed — no filesystem lookup at runtime.
• Both scripts cover every current subcommand (signup, share, remove, list, completion, --upgrade) and render flag, including sub-flags such as signup --email/--force, share --watch, remove --all/--pick/--yes/--non-interactive, and completion {bash|zsh}.
• The release workflow bundles the completions directory as gander-completions.tar.gz on every tagged release.
• README gains a "Shell completions" subsection with sourcing examples and updates the Subcommands block.

After install, users can add:

source <(gander completion bash)     # bash
eval "$(gander completion zsh)"      # zsh

Related Issue

Closes #29

Testing

• New completion_test.go covers: bash + zsh output is non-empty, expected tokens are present, unknown shells reject, no-arg rejects, and bash -n / zsh -n parse-check the generated scripts.
• go vet ./..., go test ./..., CGO_ENABLED=0 go build ./... all clean.
• Manual smoke: gander --help shows the new subcommand; gander completion bash | bash -n and the zsh equivalent both pass on this machine.

Notes

• Fish and PowerShell completions were intentionally deferred to a future issue once the Windows build lands (ci: add windows-amd64 and windows-arm64 to release matrix #31). Bash + zsh cover the majority of the install base today.
• No new dependency on cobra or similar — completion logic is a hand-written table for transparency.