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.

cli/src/castle_cli/commands/create.py

@@ -8,6 +8,7 @@ import subprocess
 from castle_cli.config import REPOS_DIR, load_config, save_config
 from castle_cli.manifest import (
     CaddySpec,
+    DefaultsSpec,
     ExposeSpec,
     HttpExposeSpec,
     HttpInternal,
@@ -92,6 +93,7 @@ def run_create(args: argparse.Namespace) -> int:
         behavior=behavior,
     )
     if behavior == "daemon":
+        prefix = name.replace("-", "_").upper()
         config.services[name] = ServiceSpec(
             id=name,
             program=name,
@@ -104,6 +106,11 @@ def run_create(args: argparse.Namespace) -> int:
             ),
             proxy=ProxySpec(caddy=CaddySpec(path_prefix=f"/{name}")),
             manage=ManageSpec(systemd=SystemdSpec()),
+            # python-fastapi scaffold reads env_prefix MY_SERVICE_ — map castle's
+            # computed port/data dir to those vars (explicit, no hidden injection).
+            defaults=DefaultsSpec(
+                env={f"{prefix}_PORT": "${port}", f"{prefix}_DATA_DIR": "${data_dir}"}
+            ),
         )
 
     save_config(config)