fix(build): declare the env the docs build reads so turbo stops stripping it

turbo runs tasks in strict env mode (2.x default): a task only sees the env
vars declared for it. `build` declared none, so every var the deploy workflow
exports was dropped before the build ran:

  NEXT_EXPORT   -> next.config gates `output: 'export'` on it. Stripped, the
                   build emits no out/, and the next workflow step is
                   `wrangler pages deploy out`.
  HANZO_DOCS_SYNC -> pre-build gates the project-docs sync on it. Stripped, the
                   sync runs — the workflow sets it to 0 precisely so the
                   committed snapshot is used instead.
  NODE_OPTIONS  -> the heap ceiling the workflow raises for this build.

`turbo run build --filter=docs --dry=json` reported envMode "strict" with
configured [] and passthrough null; it now lists NEXT_EXPORT, HANZO_DOCS_SYNC
and NODE_OPTIONS.

This regressed in 6167d746c, which moved NEXT_EXPORT and NODE_OPTIONS out of the
app's build script — where they were inline and immune to turbo's env filter —
into the workflow's env block.

The two vars that select what is built (NEXT_EXPORT, HANZO_DOCS_SYNC) belong in
`env`, where they also key the cache: a non-export build and an export build are
not interchangeable artifacts. NODE_OPTIONS only shapes how the build runs, so
it is passThroughEnv and stays out of the hash. The wildcards cover the rest of
the HANZO_DOCS_*/HANZO_SEARCH_* knobs the build scripts read, so the next one
added does not silently vanish.

turbo.json

@@ -10,6 +10,15 @@
   "tasks": {
     "build": {
       "dependsOn": ["^build"],
+      "env": [
+        "NEXT_EXPORT",
+        "GITHUB_PAGES",
+        "DOCS_SECTION",
+        "ANALYZE",
+        "HANZO_DOCS_*",
+        "HANZO_SEARCH_*"
+      ],
+      "passThroughEnv": ["NODE_OPTIONS"],
       "outputs": [
         ".output",
         "public/registry",