docs(readme): add PATH hint for ~/.cargo/bin in Quick Start

`cargo install` places the binary in ~/.cargo/bin, which may not be in
the user's PATH by default. This adds an explicit export step so new
users don't hit a "not found" error after install.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -133,6 +133,9 @@ cd zeroclaw
 cargo build --release --locked
 cargo install --path . --force --locked
 
+# Ensure ~/.cargo/bin is in your PATH
+export PATH="$HOME/.cargo/bin:$PATH"
+
 # Quick setup (no prompts)
 zeroclaw onboard --api-key sk-... --provider openrouter