[Docs] README "Development" section is missing a prerequisite list

Summary

The README's ## Development section opens with make prepare, but neither the README nor CONTRIBUTING.md lists the prerequisites a contributor needs to have installed first. New contributors hit one of:

• make: command not found (Windows users without WSL or a build-tools install)
• uv: command not found (no link in the dev section to the install instructions used in the user-install section)
• make build-web failing because Node.js / npm aren't installed (mentioned only inline in the build-web bullet)
• on Windows, the agent's Shell tool refuses to start because Git Bash isn't installed (src/kimi_cli/utils/environment.py's GitBashNotFoundError — easy to hit while testing your changes)
• general confusion about whether you pip install something or rely on uv to manage the venv

Proposed fix

Add a "Prerequisites" subsection at the top of ## Development (and ideally mirror it in CONTRIBUTING.md):

### Prerequisites

- **uv** ≥ 0.8 — package + venv manager. Install: https://docs.astral.sh/uv/getting-started/installation/
- **Python 3.14** — pinned by `.python-version`; uv will install it for you on first `uv sync` if missing. (`pyproject.toml` accepts `>=3.12`, but 3.14 matches the lockfile / CI environment.)
- **make** — runs the build/test targets in `Makefile`. Most Linux/macOS already have it; Windows users need WSL or a `make` from MSYS2 / chocolatey.
- **Node.js ≥ 20 + npm** — only required if you'll run `make build-web` or `make build` (the latter embeds the web UI).
- **Git Bash (Windows only)** — kimi-cli's agent requires `bash.exe` from Git for Windows; without it, the Shell tool will not start. Install via https://git-scm.com/download/win.
- **Nix (optional)** — a `flake.nix` is provided. `nix develop` drops you into a shell with all of the above.

Then make the existing make prepare line the second step.

Why

Lowers contributor friction. Today, new contributors have to read the Makefile, pyproject.toml, .python-version, and src/kimi_cli/utils/environment.py to figure out what they need — and the Windows + Git Bash requirement is genuinely surprising the first time you hit it.

Start with the README's ## Development section and compare its setup steps with CONTRIBUTING.md. Check the Makefile, pyproject.toml, .python-version, and src/kimi_cli/utils/environment.py references named in the issue. Done means both contributor guides clearly list the prerequisites, platform-specific Git Bash note, and the existing make prepare step in the right order.