Add shell completions (CLI subcommand + nix packaging)

[antono]

Summary

• New codanna completions <bash|zsh|fish|elvish|powershell> subcommand, generated from the clap definitions via clap_complete so it stays in sync with the CLI automatically.
• The Nix flake package now ships bash/zsh/fish completions (installShellFiles + postInstall), so they're installed automatically with the package — no extra config for home-manager / NixOS users.
• Surfaces the previously-undocumented profile command (and the new completions) in the custom help text.

Details

• completions is emitted immediately after argument parsing — before any config/index/provider setup — so it works without an initialized project.
• Bumps rust-overlay in flake.lock to a rustc 1.95 revision. This is required because the committed Cargo.lock pins sysinfo 0.39.2 (needs rustc ≥1.95); the flake previously resolved to 1.94 and failed to build.

Testing

• cargo check and cargo fmt --all clean.
• Built .#packages.default with the committed flake.lock; confirmed completions ship to:
  • share/bash-completion/completions/codanna.bash
  • share/zsh/site-functions/_codanna
  • share/fish/vendor_completions.d/codanna.fish
• Sourced the generated bash script; top-level commands and retrieve subcommands complete correctly.

Usage

codanna completions bash > ~/.local/share/bash-completion/completions/codanna
source <(codanna completions bash)   # current shell only

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