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.

packages/core-shared/AGENTS.md

@@ -20,8 +20,9 @@ Generic, reusable primitives with **zero business knowledge**. This package is t
 From `package.json`:
 - `.` — all utilities, Payload exports, tRPC init
 - `./payload` — Payload field/hook/block utilities only
-- `./trpc/init` — tRPC initialization only
+- `./trpc/init` — tRPC `initTRPC` instance + builders. **Plan 9:** also exports `t` (the raw `initTRPC.create({...})` instance) so feature packages can build their own procedures via `t.procedure.use(...)`
 - `./trpc/context` — tRPC context factory only
+- `./trpc/define-error-middleware` — **Plan 9:** factory that builds a tRPC middleware translating domain errors to `TRPCError`. Takes `ReadonlyArray<readonly [ErrorCtor, TRPC_ERROR_CODE_KEY]>` tuples; uses `instanceof` discrimination; preserves the original error as `.cause`. **Owned by features:** each feature passes its own constructors in via `integrations/api/procedures.ts`. core-shared never enumerates feature-specific error classes — this stays boundary-clean
 
 ## Test conventions