feat: Release completions as a tarball and validate loaded scripts

[razor-x]

Follow-up to #594.

Summary

• Release the shell completion loaders as a single seam-completions-v<version>.tar.gz asset instead of three bare files, and source the seam-bin AUR package from that tarball. The versioned filename also removes the need to rename the sources in the PKGBUILD.
• Harden the loaders: the output of seam completion <shell> is evaluated only when it starts with the exact first line the script generator produces. This fixes the reported fish failure, where a seam without the completion command printed Not logged in. Please run "seam login" to stdout and the loader piped it into source (fish: Unknown command: Not). Nothing the CLI prints other than a completion script is ever evaluated now.
• A test pins the sentinel first lines to the generator output so the loaders and the scripts cannot drift apart.
• Document what each shell needs before installed completions appear: bash reads them via the bash-completion package, and zsh caches its completion functions in ~/.zcompdump, which frameworks calling compinit -C never rebuild on their own. Fish needs nothing extra.

Verification

• All three hardened loaders verified in real shells against the current CLI: bash registers and completes, zsh renders the menu on the first TAB through fpath autoload in a pty, fish completes commands and enum values.
• All three verified against a fake old seam that prints Not logged in and exits 0: no errors, nothing evaluated, shells degrade to their default completion.
• Tarball creation, checksums, and extraction simulated exactly as _build.yml and makepkg run them.
• 103 tests, lint, and typecheck pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_016eYkGybhEJJE3FkaqXdwLv

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