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/fields.tsx

@@ -71,6 +71,12 @@ export function useEnvSecrets(initial: Record<string, string>) {
     <div className="space-y-4">
       <Field label="Environment">
         <div className="space-y-2">
+          <p className="text-xs text-[var(--muted)]">
+            Use <code className="font-mono">${"{port}"}</code>,{" "}
+            <code className="font-mono">${"{data_dir}"}</code>,{" "}
+            <code className="font-mono">${"{name}"}</code> for castle's computed values,
+            and <code className="font-mono">${"{secret:NAME}"}</code> for secrets.
+          </p>
           {Object.entries(env).map(([key, val]) => (
             <div key={key} className="flex items-center gap-2">
               <input value={key} readOnly className={`w-56 ${INPUT} text-xs text-[var(--muted)]`} />