fix(install): build gsd-sdk from in-repo sdk/ source, not stale npm package

PR #2386 v1 installed the published @gsd-build/sdk from npm, which ships an
older version that lacks query handlers needed by current workflows. Every
GSD release would drift further from what the installer put on PATH.

This commit rewires installSdkIfNeeded() to build from the in-repo sdk/
source tree instead:

  1. cd sdk && npm install     (build-time deps incl. tsc)
  2. npm run build             (tsc → sdk/dist/)
  3. npm install -g .          (global install; gsd-sdk on PATH)

Each step is a hard gate — failures warn loudly and point users at the
manual equivalent command. No more silent drift between installed SDK and
the rest of the GSD system.

Root package.json `files` now ships sdk/src, sdk/prompts, sdk/package.json,
sdk/package-lock.json, and sdk/tsconfig.json so npm-registry installs also
carry the source tree needed to build gsd-sdk locally.

Also fixes a blocking tsc error in sdk/src/event-stream.ts:313 — the cast
to `Array<{ type: string; [key: string]: unknown }>` needed a double-cast
via `unknown` because BetaContentBlock's variants don't carry an index
signature. Runtime-neutral type-widening; sdk vitest suite unchanged
(1256 passing; the lone failure is a pre-existing integration test that
requires external API access).

Updates the #1657/#2385 regression test to assert the new build-from-source
path (path.resolve(__dirname, '..', 'sdk') + `npm run build` + `npm install
-g .`) plus a new assertion that root package.json files array ships sdk
source.

Refs #2385

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -11,7 +11,12 @@
     "get-shit-done",
     "agents",
     "hooks",
-    "scripts"
+    "scripts",
+    "sdk/src",
+    "sdk/prompts",
+    "sdk/package.json",
+    "sdk/package-lock.json",
+    "sdk/tsconfig.json"
   ],
   "keywords": [
     "claude",