refactor(env): explicit defaults.env with placeholders; drop auto-injection

A service/job's env is now exactly its defaults.env — castle injects no hidden
convention vars. Values support ${port}/${data_dir}/${name} placeholders
(resolved at deploy, alongside ${secret:…}), so a program's own env var names
map to castle's computed values without hardcoding.

Why: the auto-injected <PREFIX>_PORT/<PREFIX>_DATA_DIR were a guess at the
program's env names — right for castle-scaffolded services, dead weight for
adopted ones (lakehouse carried two dead vars; notification-bridge/backup jobs
too). They also weren't visible in the config editor (computed at deploy), which
was the source of the 'four env vars but the UI shows none' mystery.

- core: resolve_env_vars gains a context (${port}/${data_dir}/${name});
  deploy builds env from defaults.env only — no <PREFIX>_* injection, no
  port_env. Removed the port_env field and the dead _env_prefix helper.
- cli: 'service/job create' gains repeatable --env KEY=VALUE (replaces
  --port-env); 'program create' scaffolds <PREFIX>_PORT/_DATA_DIR: ${…} for new
  daemons.
- app: removed the 'Port env' field; the Environment editor (defaults.env) is
  the single place, with a placeholder hint.
- live migration: central-context/castle-api/power-graph/protonmail mapped their
  real vars explicitly; lakehouse → just LAKEHOUSED_DAEMON_PORT: ${port}, data
  stays in ~/.lakehoused. Verified all services healthy on their ports, dead
  vars gone, zero failed units.
- docs: registry.md/design.md/stack guides + findings updated to the explicit
  model.

core 94 / cli 24 / api 52 green; ruff + app build clean.

app/src/components/detail/ServiceFields.tsx

@@ -28,7 +28,6 @@ export function ServiceFields({ service, onSave, onDelete }: Props) {
     (run.program as string) ?? ((run.argv as string[]) ?? []).join(" "),
   )
   const [port, setPort] = useState(internal.port != null ? String(internal.port) : "")
-  const [portEnv, setPortEnv] = useState((internal.port_env as string) ?? "")
   const [health, setHealth] = useState((httpExpose.health_path as string) ?? "")
   const [proxyPath, setProxyPath] = useState((caddy.path_prefix as string) ?? "")
   const [proxyHost, setProxyHost] = useState((caddy.host as string) ?? "")
@@ -55,10 +54,7 @@ export function ServiceFields({ service, onSave, onDelete }: Props) {
       if (port) {
         config.expose = {
           http: {
-            internal: {
-              port: parseInt(port, 10),
-              ...(portEnv ? { port_env: portEnv } : {}),
-            },
+            internal: { port: parseInt(port, 10) },
             ...(health ? { health_path: health } : {}),
           },
         }
@@ -101,14 +97,6 @@ export function ServiceFields({ service, onSave, onDelete }: Props) {
         />
       </Field>
       <TextField label="Port" value={port} onChange={setPort} width="w-32" mono placeholder="9001" />
-      <TextField
-        label="Port env"
-        value={portEnv}
-        onChange={setPortEnv}
-        width="w-64"
-        mono
-        placeholder="(only if the program reads a custom var)"
-      />
       <TextField label="Health path" value={health} onChange={setHealth} width="w-48" mono placeholder="/health" />
       <TextField label="Proxy path" value={proxyPath} onChange={setProxyPath} width="w-48" mono placeholder="/my-service" />
       <TextField label="Proxy host" value={proxyHost} onChange={setProxyHost} mono placeholder="my-service.lan" />