feat(runtime): ✨ support bunx binary downloads

[CarmeloCampos]

Why

opencode-worktree currently relies on postinstall to download its platform binary. Package runners such as bunx can skip lifecycle scripts, so bunx opencode-worktree installs the package but fails because the executable is missing.

Changes

• Download the platform binary from the launcher when it is absent.
• Share binary resolution and download logic between the launcher and postinstall.
• Preserve postinstall as an optimization for regular global installs.
• Document bunx opencode-worktree.

Benefits

• Run without a global installation: bunx opencode-worktree.
• Works when lifecycle scripts are skipped by package runners, including npx and pnpx.
• Keeps existing npm i -g opencode-worktree behavior unchanged.
• Avoids duplicated platform/download logic.

Verification

• Node syntax checks pass for the launcher, runtime module, and postinstall script.
• npm pack --dry-run confirms the runtime module is included in the published package.
• git diff --check passes.