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.

pyproject.toml

@@ -1,12 +1,12 @@
 [project]
-name = "castle-workspace"
+name = "wildpc-workspace"
 version = "0.0.0"
-description = "Castle platform workspace root"
+description = "Wild PC platform workspace root"
 requires-python = ">=3.11"
 
 [tool.uv.workspace]
-members = ["core", "cli", "castle-api"]
+members = ["core", "cli", "wildpc-api"]
 
 [tool.uv.sources]
-castle-core = { workspace = true }
-castle-cli = { workspace = true }
+wildpc-core = { workspace = true }
+wildpc-cli = { workspace = true }