feat: guarded plugin auto-detection — installing @soulcraft/cor is the opt-in

With plugins unset (the default), init() probes for the first-party
accelerator: not installed means plain brainy with zero noise; installed and
healthy means it loads and announces itself; installed but broken (import
failure, invalid shape, failed activation, version mismatch) makes init()
THROW. An installed accelerator never silently vanishes behind the JS engines
- the anti-drift posture of the explicit list, applied to detection.

plugins: []/false stays a true opt-out (no probe); an explicit list keeps its
required-and-loud semantics. The import runs through an importPluginPackage
seam (variable specifier - bundlers cannot static-resolve the optional
package; tests simulate all outcomes without it installed).

Also: when a plugin activates but registers zero native providers (e.g. a
licensing gate declining to engage), the provider summary now warns loudly
instead of leaving every query silently on the JS engines.

Supersedes 8.0.8's explicit-opt-in wording; README/PLUGINS/types now document
the guarded contract. 7 new tests (tests/unit/plugin-autodetect.test.ts).

src/types/brainy.types.ts

@@ -1804,10 +1804,13 @@ export interface BrainyConfig {
   eagerEmbeddings?: boolean
 
   // Plugin configuration
-  // Controls which plugins are loaded during init(). Plugins are explicit
-  // opt-in — brainy NEVER auto-imports a package that isn't listed here.
-  // - undefined (default): no plugins loaded (same as [] / false)
-  // - false / []: no plugins loaded
+  // Controls which plugins are loaded during init().
+  // - undefined (default): guarded auto-detection of the first-party
+  //   accelerator (@soulcraft/cor) — installing the package IS the opt-in.
+  //   Not installed → plain brainy, silently. Installed → it loads and
+  //   announces itself. Installed but broken → init() THROWS (an installed
+  //   accelerator never silently vanishes behind the JS engines).
+  // - false / []: no plugins, no detection (explicit opt-out)
   // - ['@soulcraft/cor']: load exactly these packages; a listed plugin that
   //   fails to load or is invalid THROWS (loud, never a silent JS fallback)
   plugins?: string[] | false