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/python-cli.md

@@ -1,9 +1,9 @@
-# Python Tools in Castle
+# Python Tools in Wild PC
 
 > **This is a stack — creation-time guidance for writing _new_ CLI tools.**
-> A stack is a template + conventions, not a runtime requirement. `castle program create
+> A stack is a template + conventions, not a runtime requirement. `wildpc program create
 > --stack python-cli` scaffolds from it and seeds the program's default dev-verb
-> commands. An existing CLI adopted with `castle program add` doesn't need this stack — it
+> commands. An existing CLI adopted with `wildpc program add` doesn't need this stack — it
 > declares its own `commands:`. See @docs/registry.md for `commands:`,
 > `stack:` (optional), and `repo:`.
 
@@ -49,11 +49,11 @@ Examples: `code/pdf2md/`, `code/gpt/`, `code/protonmail/`
 ## Creating a new tool
 
 ```bash
-castle program create my-tool --stack python-cli --description "Does something"
+wildpc program create my-tool --stack python-cli --description "Does something"
 cd /data/repos/my-tool && uv sync
 ```
 
-This scaffolds the project and registers it in `castle.yaml`.
+This scaffolds the project and registers it in `wildpc.yaml`.
 
 ## pyproject.toml