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.

core/src/castle_core/manifest.py

@@ -111,12 +111,6 @@ class HttpInternal(BaseModel):
     host: str = "127.0.0.1"
     port: int = Field(ge=1, le=65535)
     unix_socket: str | None = None
-    # Env var the program actually reads for its bind port. Castle's convention
-    # injects <PREFIX>_PORT, but an adopted program may read a different name
-    # (e.g. lakehoused reads LAKEHOUSED_DAEMON_PORT). When set, deploy also sets
-    # this var to `port`, so castle genuinely drives the bind instead of relying
-    # on the program's default happening to match.
-    port_env: str | None = None
 
 
 class HttpPublic(BaseModel):