Add castle doctor + getting-started docs

The README/docs explained how the system works but not how to get it up
and running, and there was no way to tell a healthy node from a
half-configured one. Two additions close that gap.

castle doctor — a read-only preflight/postflight check. It inspects
setup *and* runtime (CLI on PATH, uv, lingering; repo:, control plane
registered, dashboard built; gateway/api running + listening, specs
generated; and — under tls=acme — DNS-plugin Caddy, provider token,
:443 bind; tunnel config for public services) and, for anything not
green, prints the exact next command. Exit 0 when nothing failed
(warnings allowed), 1 otherwise, so it doubles as a scriptable smoke
test after install/deploy. Agents can lean on it the same way they use
`castle tool list`.

Docs — the quick start now leads with prerequisites and one command
(install.sh installs the CLI + registers the control plane, so the old
manual `uv tool install` step is gone), adds a `castle doctor` verify
step, and gains an "exposure ladder" table framing the three rungs
(localhost → LAN HTTPS → public) that link the deep DNS/TLS/tunnel docs.
install.sh's closing summary and the AGENTS.md CLI reference mention
doctor too.

cli/src/castle_cli/main.py

@@ -193,6 +193,9 @@ def build_parser() -> argparse.ArgumentParser:
     subparsers.add_parser("stop", help="Stop all services and the gateway")
     subparsers.add_parser("restart", help="Restart all services and jobs")
     subparsers.add_parser("status", help="Show status across the platform")
+    subparsers.add_parser(
+        "doctor", help="Diagnose setup + runtime health, with next-step hints"
+    )
     p = subparsers.add_parser("deploy", help="Apply config to runtime (units + Caddyfile)")
     p.add_argument("name", nargs="?", help="Service/job to deploy (default: all)")
 
@@ -346,6 +349,10 @@ def main() -> int:
         from castle_cli.commands.service import run_status
 
         return run_status(args)
+    if cmd == "doctor":
+        from castle_cli.commands.doctor import run_doctor
+
+        return run_doctor(args)
     if cmd == "deploy":
         from castle_cli.commands.deploy import run_deploy