Rename Castle -> Wild PC across the repo

Repo-side rename only (Phases 1-3 of the migration plan); the live box
(~/.castle, systemd units, /data/castle, domains) is a separate cutover.

- Slug `castle` -> `wildpc`: CLI command, module names (wildpc_core/cli/api),
  dist names, entry point `wildpc = wildpc_cli.main:main`.
- Identifiers: CastleConfig/NATSClient/DirError/MDNS -> Wildpc*.
- Env/constants: CASTLE_* -> WILDPC_*; ~/.castle -> ~/.wildpc, castle.yaml ->
  wildpc.yaml, /data/castle -> /data/wildpc.
- Systemd UNIT_PREFIX castle- -> wildpc-; own programs castle-api/gateway/etc.
- Display prose "Castle" -> "Wild PC" in docs, agent-guide files, README, frontend.
- Package dirs and bootstrap yaml renamed via git mv; lockfiles regenerated;
  redundant nested uv.lock files dropped (workspace root lock is authoritative).

Tests: core 273, cli 47, wildpc-api 120 all pass. Frontend type-checks + builds.
Fixed a stale test fixture (secret_env_path kind arg) broken pre-rename.

docs/stacks/hugo.md

@@ -1,13 +1,13 @@
-# Hugo static sites in Castle
+# Hugo static sites in Wild PC
 
 > **This is a stack — creation-time guidance for writing _new_ sites.**
-> A stack is a template + conventions, not a runtime requirement. `castle program
+> A stack is a template + conventions, not a runtime requirement. `wildpc program
 > create --stack hugo` scaffolds from it (via Hugo's own `hugo new site`) and seeds
-> the program's default build verb. An existing Hugo site adopted with `castle
+> the program's default build verb. An existing Hugo site adopted with `wildpc
 > program add` doesn't need this stack — it declares its own `commands:` /
 > `build:`. See @docs/registry.md for `commands:`, `stack:` (optional), and `repo:`.
 
-How to build, serve, and manage [Hugo](https://gohugo.io) sites as castle programs.
+How to build, serve, and manage [Hugo](https://gohugo.io) sites as wildpc programs.
 
 ## Stack
 
@@ -18,27 +18,27 @@ How to build, serve, and manage [Hugo](https://gohugo.io) sites as castle progra
   `/posts/index.html`) and 404s missing paths. (The default `spa: true` is a
   single-page-app fallback that serves the root `index.html` for every unmatched
   path — correct for React/Vite, but it swallows a Hugo site's in-page links back
-  to the homepage. `castle program create --stack hugo` sets `spa: false` for you.)
+  to the homepage. `wildpc program create --stack hugo` sets `spa: false` for you.)
 - Package manager (only if a theme needs an asset pipeline): pnpm
 
 Hugo has one meaningful dev verb — **build**. It has no native lint/test/type-check,
-so the stack advertises only `build` / `install` / `uninstall`; `castle check` and
+so the stack advertises only `build` / `install` / `uninstall`; `wildpc check` and
 friends aren't offered (a site can still declare its own, e.g. an HTML linter, under
 `commands:` — a declared verb always wins over the stack).
 
 ## Create a new site
 
 ```bash
-castle program create my-site --stack hugo --description "My site"
+wildpc program create my-site --stack hugo --description "My site"
 cd /data/repos/my-site
-castle program build my-site      # hugo --gc --minify -> public/
-castle apply my-site              # serve at my-site.<gateway.domain>
+wildpc program build my-site      # hugo --gc --minify -> public/
+wildpc apply my-site              # serve at my-site.<gateway.domain>
 ```
 
 The scaffold delegates the canonical skeleton to `hugo new site` (archetypes/,
 content/, layouts/, static/, themes/, hugo.toml) and overlays the pieces a bare
 skeleton lacks: minimal `layouts/` so the site **builds and serves without a
-theme**, an example `content/posts/hello.md`, a castle-flavored `hugo.toml`
+theme**, an example `content/posts/hello.md`, a wildpc-flavored `hugo.toml`
 (`baseURL = "/"`, so assets resolve at the root of the site's own subdomain), and a
 `.gitignore` for the regenerated `public/` and `resources/`.
 
@@ -78,14 +78,14 @@ cd themes/<theme> && pnpm install
 
 ## Deployment shape
 
-`castle program create --stack hugo` writes:
+`wildpc program create --stack hugo` writes:
 
 - **`programs/<name>.yaml`** — `source`, `stack: hugo`, `build.outputs: [public]`.
 - **`deployments/statics/<name>.yaml`** — `manager: caddy`, `root: public`,
   `reach: internal` (flip to `public` to also expose over the tunnel).
 
 The gateway serves `<source>/public` in place — no copy, no Node/Hugo process at
-runtime. `castle program build` regenerates `public/`; `castle apply` renders the
+runtime. `wildpc program build` regenerates `public/`; `wildpc apply` renders the
 route and reloads the gateway.
 
 ## Adopting an existing Hugo site
@@ -93,7 +93,7 @@ route and reloads the gateway.
 No stack needed — adopt the repo and declare how it builds:
 
 ```bash
-castle program add /path/to/site --name my-site
+wildpc program add /path/to/site --name my-site
 ```
 
 Then set `build.commands` (as above) and add a `manager: caddy` deployment. This is