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.

app/src/components/detail/ProgramFields.tsx

@@ -95,7 +95,7 @@ export function ProgramFields({ program, onSave, onDelete }: Props) {
         onChange={setSource}
         mono
         placeholder="/data/repos/my-prog"
-        hint="The working copy on disk. Castle runs dev verbs and builds here. Absolute path, or repo:<name> for castle's own programs."
+        hint="The working copy on disk. Wild PC runs dev verbs and builds here. Absolute path, or repo:<name> for wildpc's own programs."
       />
       <Field
         label="Stack"
@@ -119,7 +119,7 @@ export function ProgramFields({ program, onSave, onDelete }: Props) {
         onChange={setRepo}
         mono
         placeholder="https://github.com/me/x.git"
-        hint="Git URL so 'castle program clone' can fetch the source on a fresh machine. An existing working copy at Source takes precedence."
+        hint="Git URL so 'wildpc program clone' can fetch the source on a fresh machine. An existing working copy at Source takes precedence."
       />
       <TextField
         label="Ref"
@@ -134,7 +134,7 @@ export function ProgramFields({ program, onSave, onDelete }: Props) {
         value={deps}
         onChange={setDeps}
         placeholder="pandoc, poppler-utils"
-        hint="OS packages the program needs. Listed for reference — castle does not install them."
+        hint="OS packages the program needs. Listed for reference — wildpc does not install them."
       />
 
       <Field