feat: Add one-line shell installer following vipune pattern

[randomm]

Motivation

Currently users must manually download .tar.xz archives from GitHub Releases and extract the binary manually. vipune has a simpler one-line install pattern:

curl --proto '=https' --tlsv1.2 -LsSf https://github.com/randomm/vipune/releases/latest/download/vipune-installer.sh | sh

This improves user experience by providing a familiar, frictionless installation method.

User Story

As an AI agent developer installing oo, I want a one-line install command so that I can quickly get started without manually downloading and extracting archives.

Proposed Solution

Enable cargo-dist's shell installer generation by updating Cargo.toml:

[workspace.metadata.dist]
installers = ["shell"]  # ← Change from empty
install-path = "CARGO_HOME"  # ← Recommended addition

cargo-dist will then auto-generate oo-installer.sh as part of the existing release pipeline. The installer will:

• Detect OS/architecture automatically (Apple Silicon macOS, ARM64 Linux, x64 Linux)
• Download the correct binary from GitHub Releases
• Verify SHA256 checksum
• Install to $CARGO_HOME/bin (or $HOME/.cargo/bin)
• Configure PATH if needed

Release flow remains unchanged:

1. Merge to main → release-plz creates version bump PR
2. Merge that PR → release-plz creates git tag + publishes to crates.io
3. Tag push triggers cargo-dist → builds binaries + generates installer + creates GitHub Release

Acceptance Criteria

• installers = ["shell"] and install-path = "CARGO_HOME" added to Cargo.toml
• Next release includes oo-installer.sh artifact in GitHub Release
• Installer successfully installs oo on current target platforms (aarch64-apple-darwin, aarch64-unknown-linux-gnu, x86_64-unknown-linux-gnu)
• README.md updated with one-line install example
• All existing tests pass (~197 tests)
• CI workflow green after merge

Definition of Done

• Tests written and passing (verification tests for installer generation)
• 80%+ coverage for new code (minimal changes expected)
• Linting passes (cargo fmt --check, cargo clippy -- -D warnings)
• Documentation updated (README.md with install instructions)
• Release verified to include installer artifact

[randomm]

/label ready