docs: add checksum verification commands to release install instructions

[puremachinery]

Context

The README install section says:

Verify the download against the matching release `SHA256SUMS` before moving it into `PATH`.

But it does not show the actual commands to do this. Since releases are now live (v0.1.0+), users need copy-pasteable verification steps.

Proposed change

Add the platform-specific verification commands after the download step in the README:

macOS/Linux

```bash
curl -LO "https://github.com/puremachinery/gemini-cli-go/releases/download/\${VERSION}/SHA256SUMS"
sha256sum -c SHA256SUMS --ignore-missing

or on macOS: shasum -a 256 -c SHA256SUMS --ignore-missing

```

Windows (PowerShell)

```powershell
(Get-FileHash gemini-cli-go_*_windows_amd64.zip -Algorithm SHA256).Hash

Compare with the matching line in SHA256SUMS

```

Adjust the exact commands to match the actual `SHA256SUMS` format published by the release workflow.

Why this is a good first issue

• Markdown-only change in README.md
• No Go code involved
• Just needs someone to verify the commands work against an actual release artifact

Relevant files

• `README.md` — install section
• `scripts/release-build.sh` — generates SHA256SUMS (check format)