feat: enhance completions with 'fabric-ai' alias, dynamic exec, installer

CHANGES
- Support 'fabric-ai' alias across Zsh, Bash, and Fish
- Use invoked command for dynamic completion list queries
- Refactor Fish completions into reusable registrar for multiple commands
- Update Bash completion to reference executable via COMP_WORDS[0]
- Extend Zsh compdef to register fabric and fabric-ai
- Add cross-shell installer script with autodetection and dry-run mode
- Document installation, features, troubleshooting in new completions guide

docs/Shell-Completions.md

@@ -0,0 +1,124 @@
+# Shell Completions for Fabric
+
+Fabric comes with shell completion support for Zsh, Bash, and Fish shells. These completions provide intelligent tab-completion for commands, flags, patterns, models, contexts, and more.
+
+## Quick Setup (Automated)
+
+For a quick automated installation, use the setup script:
+
+```bash
+# Run the automated setup script
+./completions/setup-completions.sh
+
+# Or see what it would do first
+./completions/setup-completions.sh --dry-run
+```
+
+The script will:
+
+- Detect whether you have `fabric` or `fabric-ai` installed
+- Detect your current shell (zsh, bash, or fish)
+- Use your existing `$fpath` directories (for zsh) or standard completion directories
+- Install the completion file with the correct name
+- Provide instructions for enabling the completions
+
+For manual installation or troubleshooting, see the detailed instructions below.
+
+## Manual Installation
+
+### Zsh
+
+1. Copy the completion file to a directory in your `$fpath`:
+
+   ```bash
+   sudo cp completions/_fabric /usr/local/share/zsh/site-functions/
+   ```
+
+2. **Important**: If you installed fabric as `fabric-ai`, create a symlink so completions work:
+
+   ```bash
+   sudo ln -s /usr/local/share/zsh/site-functions/_fabric /usr/local/share/zsh/site-functions/_fabric-ai
+   ```
+
+3. Restart your shell or reload completions:
+
+   ```bash
+   autoload -U compinit && compinit
+   ```
+
+### Bash
+
+1. Copy the completion file to a standard completion directory:
+
+   ```bash
+   # System-wide installation
+   sudo cp completions/fabric.bash /etc/bash_completion.d/
+
+   # Or user-specific installation
+   mkdir -p ~/.local/share/bash-completion/completions/
+   cp completions/fabric.bash ~/.local/share/bash-completion/completions/fabric
+   ```
+
+2. **Important**: If you installed fabric as `fabric-ai`, create a symlink:
+
+   ```bash
+   # For system-wide installation
+   sudo ln -s /etc/bash_completion.d/fabric.bash /etc/bash_completion.d/fabric-ai.bash
+
+   # Or for user-specific installation
+   ln -s ~/.local/share/bash-completion/completions/fabric ~/.local/share/bash-completion/completions/fabric-ai
+   ```
+
+3. Restart your shell or source the completion:
+
+   ```bash
+   source ~/.bashrc
+   ```
+
+### Fish
+
+1. Copy the completion file to Fish's completion directory:
+
+   ```bash
+   mkdir -p ~/.config/fish/completions
+   cp completions/fabric.fish ~/.config/fish/completions/
+   ```
+
+2. **Important**: If you installed fabric as `fabric-ai`, create a symlink:
+
+   ```bash
+   ln -s ~/.config/fish/completions/fabric.fish ~/.config/fish/completions/fabric-ai.fish
+   ```
+
+3. Fish will automatically load the completions (no restart needed).
+
+## Features
+
+The completions provide intelligent suggestions for:
+
+- **Patterns**: Tab-complete available patterns with `-p` or `--pattern`
+- **Models**: Tab-complete available models with `-m` or `--model`
+- **Contexts**: Tab-complete contexts for context-related flags
+- **Sessions**: Tab-complete sessions for session-related flags
+- **Strategies**: Tab-complete available strategies
+- **Extensions**: Tab-complete registered extensions
+- **Gemini Voices**: Tab-complete TTS voices for `--voice`
+- **File paths**: Smart file completion for attachment, output, and config options
+- **Flag completion**: All available command-line flags and options
+
+## Alternative Installation Method
+
+You can also source the completion files directly in your shell's configuration file:
+
+- **Zsh**: Add to `~/.zshrc`: `source /path/to/fabric/completions/_fabric`
+- **Bash**: Add to `~/.bashrc`: `source /path/to/fabric/completions/fabric.bash`
+- **Fish**: The file-based installation method above is preferred for Fish
+
+## Troubleshooting
+
+- If completions don't work, ensure the completion files have proper permissions
+- For Zsh, verify that the completion directory is in your `$fpath`
+- If you renamed the fabric binary, make sure to create the appropriate symlinks as described above
+- Restart your shell after installation to ensure completions are loaded
+
+The completion system dynamically queries the fabric command for current patterns, models, and other resources, so your completions will always be up-to-date with your fabric installation.