docs: configure claude-code input in home-manager flake example

[jx-wi]

What & why

Checklist

CI runs nix flake check (host-side guarantees, guest build, shellcheck) automatically. The
full-boot smoke test (tests/boot.sh) needs a real VM and is not in CI — so if your change
touches the guest, wrapper, or boot path, you must run it locally on a Nix+KVM box and paste
the result here. (See CLAUDE.md → "Definition of done".)

• nix flake check is green (CI on this PR, or run locally)
• Ran bash tests/boot.sh on a Nix+KVM box — pasted the N passed, M failed line below; or N/A (docs / CI-only change, no guest/wrapper/boot impact)
• Touches the TTY (zsh/ZLE/terminfo/ssh -tt)? Did a human ccvm --shell pass — resize, vim, less, vi-mode — since terminal fidelity isn't automated
• Security invariants still hold (no secret to disk/argv/seed; host key pinned; only the CWD shared) — see CLAUDE.md "Security invariants"
• Commit trailer is the exact ccvm form: Co-authored-by: Claude <noreply@anthropic.com>