docs+install: sweep deploy/start → castle apply

Update the README, AGENTS.md, install.sh, and docs/ for the converge
model: one `castle apply [name] [--plan]` replaces `deploy && start` and
the per-kind enable/disable/install/start/stop verbs. CLI references now
list apply + restart (the imperative bounce) + logs; recipes use
`castle apply`; "turn it off" is documented as `enabled: false` + apply.
install.sh's next-steps collapse to a single `castle apply`.

Verified: no retired-verb command instructions remain in any markdown;
install.sh syntax OK; live `castle apply --plan` reports converged;
doctor all-green; core 129 / cli 31 / api 59 pass.

docs/developing-castle.md

@@ -63,7 +63,7 @@ for tools/libraries.
   at `/usr/local/bin/caddy` when `tls: acme`.
 - Systemd user units at `~/.config/systemd/user/castle-*.service` (+ `.timer`);
   the unit for program `X` is `castle-X.service`. Use drop-in `*.service.d/*.conf`
-  for extra env `castle deploy` shouldn't overwrite.
+  for extra env `castle apply` shouldn't overwrite.
 - The `container` launcher resolves docker via `shutil.which("docker")` (preferred
   over rootless podman on this box).
 - Service data at `$CASTLE_DATA_DIR/<name>/`; secrets at `~/.castle/secrets/`