docs(plan-9): doc-pass slice 1 — CLAUDE.md, core-shared AGENTS, architecture, plan-8 annotations

First slice of the combined Plan 8 + Plan 9 doc-update pass:
- CLAUDE.md Key Conventions: append schema-in-use-case, presenter,
  controller unknown input, feature-scoped tRPC error mapping, public
  surface split (./ui)
- packages/core-shared/AGENTS.md: document defineErrorMiddleware export
  + t re-export from trpc/init
- docs/superpowers/plans/2026-05-05-plan-8-*.md and matching spec:
  one-line note that some controller/router patterns shifted in Plan 9;
  link to the Plan 9 refactor log
- docs/architecture/overview.md: data-flow box now shows xProcedure +
  xInputSchema + xOutputSchema.parse + presenter + middleware lanes;
  three explanatory paragraphs added (schemas, presenter, error mapping)
- docs/architecture/dependency-flow.md: app-side ./ui subpath note,
  allowed/disallowed examples updated for Plan 9 paths

Remaining doc-pass items (root AGENTS.md, per-feature AGENTS.md ×5,
core-testing AGENTS.md, adding-a-feature.md, tdd-workflow.md,
testing-strategy.md, vertical-feature-spec.md) follow in subsequent
commits — to be dispatched in parallel.

docs/architecture/dependency-flow.md

@@ -31,6 +31,10 @@
     Composition exceptions:
       core-api  → @repo/<feature>/api (subpath only)
       core-cms  → @repo/<feature>/cms (subpath only)
+
+  App-side feature subpaths (Plan 9):
+    @repo/<feature>     — contracts (types, errors, schemas, IUseCase aliases, router type, constants)
+    @repo/<feature>/ui  — UI artifacts (query builders, components)
 ```
 
 ## Concrete examples
@@ -41,6 +45,8 @@ Allowed:
 import { appRouter } from "@repo/core-api";
 import { NextTrpcProvider } from "@repo/core-trpc/next";
 import { bindProductionBlog } from "@repo/blog/di/bind-production";
+import { signInInputSchema, type SignInInput } from "@repo/auth";   // contracts (Plan 9)
+import { articleBySlugQuery } from "@repo/blog/ui";                  // queries (Plan 9)
 
 // in packages/blog
 import { slugifyIfMissing } from "@repo/core-shared/payload";
@@ -63,9 +69,17 @@ import { articles } from "@repo/blog/src/integrations/cms/collections/articles";
 
 // in packages/core-shared
 import { blogRouter } from "@repo/blog/api";         // ❌ core → feature
+import { ArticleNotFoundError } from "@repo/blog";   // ❌ core → feature
+                                                     // (defineErrorMiddleware takes Error
+                                                     // constructors as args from features —
+                                                     // core-shared never imports them)
 
 // in packages/core-trpc
 import { someBlogThing } from "@repo/blog";          // ❌ core → feature (only core-api/core-cms have exception)
+
+// in apps (using the wrong subpath)
+import { articleBySlugQuery } from "@repo/blog";     // ❌ queries live on ./ui
+import { Article } from "@repo/blog/ui";             // ❌ types live on the root subpath
 ```
 
 ## Enforcement strategy