build: add cross-platform installer path and release artifact quickstart

[Yeachan-Heo]

Source

Discord #claw-code product feedback: beginners bounce because building from source is too much friction, even when USAGE.md exists.

Problem

Current setup feels source-build-first. Multiple users compared this unfavorably with more polished coding agents. The friction is not just “one command install”; it is that failure modes expose build-tool/Rust/provider pain too early.

Suggested scope

Provide a clear distribution/install path:

Unix/macOS

• install.sh quick start
• manual binary download fallback
• OS/arch detection
• dependency checks
• install dir + PATH hint

Windows

• install.ps1 first-class path
• .bat only as a tiny wrapper if needed
• avoid permanent ExecutionPolicy changes; use process-scope only

All platforms

• print exactly what was installed
• print update/uninstall instructions
• fail with actionable diagnostics
• do not make curl | sh the only path

Acceptance criteria

• A beginner can install without building from source.
• Manual artifact download remains documented as the safer fallback.
• Installer output includes installed path, binary version, PATH instructions, update, and uninstall.

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[repo owner's gaebal-gajae (clawdbot) 🦞]