[Feature]: auto install of completions via install script

[lockwobr]

Prerequisites

• I searched existing issues

Feature Summary

• Add auto install via install script tab completions.
• Also support home brew.

Problem/Use Case

The aicr CLI already supports shell completion via aicr completion bash|zsh|fish|pwsh (urfave/cli v3), but users must manually configure it as a post-install step. Most users never discover this, so they miss out on tab completion for commands and flags.

Proposed Solution

Proposal

Make shell completion automatic for both install paths:

1. Install script (./install)

Add a setup_completions function that runs after the binary is installed:

• Detect shell from $SHELL (bash, zsh, fish)
• Write completion scripts to auto-loaded directories (file-based, not RC-based):
  • Bash: /usr/share/bash-completion/completions/aicr → fallback to ~/.local/share/bash-completion/completions/aicr
  • Zsh: /usr/local/share/zsh/site-functions/_aicr → fallback to ~/.local/share/zsh/site-functions/_aicr
  • Fish: ~/.config/fish/completions/aicr.fish
• Permission handling: try direct write, fall back to sudo, then fall back to user-local XDG directories
• Non-fatal: completion failure warns but never breaks the install (|| true)
• Opt-out: -s flag or AICR_NO_COMPLETIONS=1 env var to skip
• Uses absolute binary path ("${INSTALL_DIR}/${BIN_NAME}") since $PATH may not include the install dir yet

2. Homebrew formula (.goreleaser.yaml)

Add install and test blocks to the brews section:

    install: |
      bin.install "aicr"
      generate_completions_from_executable(bin/"aicr", "completion")
    test: |
      assert_match version.to_s, shell_output("#{bin}/aicr --version")

generate_completions_from_executable is a Homebrew built-in that handles bash, zsh, and fish completions automatically.

## 3. Documentation updates
Update `docs/user/installation.md` and `site/content/docs/user/installation.md:`
- Note that completions are now automatic
- Document the -s skip flag
- Keep manual instructions for additional shells
- Update uninstall section with completion file paths to clean up

### Success Criteria

Install script automatically sets up tab completions

### Alternatives Considered

_No response_

### Component

CLI (aicr)

### Priority

Nice to have

### Compatibility / Breaking Changes

_No response_

### Operational Considerations

_No response_

### Are you willing to contribute?

_No response_