[FEATURE]: Add standard CLI flags to install script (--help, --version, --no-modify-path) #5884

Summary

The install script (curl -fsSL https://opencode.ai/install | bash) currently lacks standard CLI flags that users expect from modern install scripts. This proposal adds basic flags following conventions established by widely-used installers.

Problem

• No --help - Users can't discover available options without reading the source code
• No --version flag - The VERSION env var works, but flags are more discoverable and ergonomic
• No --no-modify-path - The script modifies shell config files (.zshrc, .bashrc, etc.) without an opt-out, which causes issues on:
  • NixOS with home-manager (config files are read-only symlinks to /nix/store/...)
  • Immutable distros (Fedora Silverblue, etc.)
  • Declarative dotfile managers (chezmoi, yadm, GNU Stow)
• Unknown flags are silently ignored - Typos like --no-modify instead of --no-modify-path fail silently

Proposed Flags

Additionally, unknown arguments should produce a warning so users are aware of typos.

Example Usage

# Show available options
curl -fsSL https://opencode.ai/install | bash -s -- --help

# Install a specific version
curl -fsSL https://opencode.ai/install | bash -s -- --version 1.0.180

# Install without modifying shell configs (for Nix users, etc.)
curl -fsSL https://opencode.ai/install | bash -s -- --no-modify-path

Options:
  -v, --verbose           Enable verbose output
  -q, --quiet             Disable progress output
  -y                      Disable confirmation prompt
      --no-modify-path    Don't configure the PATH environment variable
  -h, --help              Print help
  -V, --version           Print version

--no-confirm              Accept defaults, non-interactive mode

Usage: [NONINTERACTIVE=1] [CI=1] install.sh [options]
    -h, --help       Display this message.

Options:
  -V, --verbose             Enable verbose output
  -f, -y, --force, --yes    Skip the confirmation prompt
  -p, --platform            Override the platform
  -b, --bin-dir             Override the bin installation directory
  -v, --version             Install a specific version
  -h, --help                Display this help message

BOOTSTRAP_HASKELL_NONINTERACTIVE  - noninteractive installation
BOOTSTRAP_HASKELL_VERBOSE         - more verbose installation
BOOTSTRAP_HASKELL_GHC_VERSION     - specific GHC version to install
BOOTSTRAP_HASKELL_ADJUST_BASHRC   - whether to adjust PATH in bashrc

Additional Notes

• All changes are backward compatible - existing usage continues to work
• Currently "no config file found" exits with an error - this could be changed to a warning, allowing the install to succeed regardless
• The VERSION environment variable can remain as an alternative to --version