Make missing-binary install hints OS-aware

[theyashgupta]

When a required binary is missing at startup, Dispatch prints an install hint. Those hints are hardcoded to macOS in src/server/bootstrap/binary-check.ts:

const INSTALL_HINTS: Record<string, string> = {
  tmux: "brew install tmux",
  ttyd: "brew install ttyd",
  claude: "install Claude Code — https://docs.claude.com/claude-code",
  git: "xcode-select --install  (or: brew install git)",
};

On Linux, brew and xcode-select are wrong.

What to do

Make the hints OS-aware using process.platform:

• On Linux, show a suitable hint (e.g. sudo apt install tmux / sudo dnf install tmux for tmux and git, and the ttyd install page for ttyd).
• Keep the current text as the macOS branch.
• claude stays the same on every platform.

How to verify

Temporarily rename one of the binaries off your PATH, start Dispatch, and confirm the printed hint matches your OS.

Why it's a good first issue

One file, one small function, and the fix is easy to confirm by eye.