docs: document `code-oz --version` as the post-install smoke check

[omerakben]

Context. The README's Install section walks through three channels (curl, npm, Homebrew) but does not show first-time users how to verify the install worked. The internal smoke scripts already use code-oz --version for exactly that purpose (example).

Ask. After the install commands, add a "Verify the install" block:

```sh
code-oz --version

v0.20.1-alpha.0

```

Why this is a good first issue. One markdown edit, no code, no test setup.

Acceptance.

• A "Verify the install" example exists in README.md immediately after the install commands
• The expected output is shown as a comment in the block