feat: install script for curl-pipe-sh installation

[auswm85]

Summary

Add an install script for quick installation via curl.

curl -sSf https://rungstack.com/install.sh | sh

Current State

Installation options exist but require extra steps:

• cargo install rung-cli (requires Rust toolchain)
• cargo binstall rung-cli (requires cargo-binstall)
• brew install auswm85/tap/rung (macOS, requires tap)
• Manual download from GitHub releases

Proposed Behavior

# Install latest
$ curl -sSf https://rungstack.com/install.sh | sh
Detected: aarch64-apple-darwin
Downloading rung v0.6.0...
Installing to /usr/local/bin/rung...
✓ rung installed successfully!

Run 'rung --help' to get started.

# Install specific version
$ curl -sSf https://rungstack.com/install.sh | sh -s -- --version v0.5.0
Downloading rung v0.5.0...
✓ rung v0.5.0 installed successfully!

Implementation

The script should:

• Detect OS and architecture
• Fetch latest release version from GitHub API (or use specified version)
• Download appropriate binary from releases
• Verify checksum (if available)
• Install to /usr/local/bin or ~/.local/bin
• Provide clear success/error messages

Platform Detection

OS: $(uname -s)      # Darwin, Linux
ARCH: $(uname -m)    # x86_64, arm64/aarch64

Target Mapping

Detected	Release Asset
Darwin + arm64	aarch64-apple-darwin
Darwin + x86_64	x86_64-apple-darwin
Linux + x86_64	x86_64-unknown-linux-gnu
Windows	Suggest manual download or scoop

Flags

Flag	Description
--version <tag>	Install specific version (e.g., v0.5.0)
--help	Show usage

Acceptance Criteria

• Script works on macOS (Intel + Apple Silicon)
• Script works on Linux (x86_64)
• --version flag to install specific version
• Graceful error handling (no binary available, network failure, invalid version)
• Respects INSTALL_DIR env var for custom location
• --help flag shows usage
• Add to README.md installation section