docs: strip residual Phase/Plan refs from guides

Five spots across three guides referenced the original implementation
phasing (Plan-N / Phase-N nomenclature) from the template's setup era.
Now that the template-reset epic completed and the setup history was
archived, these refs are dead vocabulary for fresh consumers.

guides/scaffolding-a-feature.md:
  - "real repo body is a Phase-1 stub" -> "real repo body is a stub"
  - "## Phase-1 scope (intentionally limited)" -> "## Scope (intentionally limited)"
  - "manually authored as part of Phase-2 wiring" -> "manually authored
    as part of the post-scaffold wiring"

guides/adding-a-feature.md:
  - "the generator's Phase-1 scope doesn't fit" -> "the generator's
    default scope doesn't fit"

guides/tdd-workflow.md:
  - "**File naming convention (post-Plan-8):**" -> "**File naming
    convention:**"

The Plan-9 refs in docs/architecture/data-flow-explainer.html will be
handled separately along with that file's other staleness in the
next commit.

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

docs/guides/tdd-workflow.md

@@ -605,7 +605,7 @@ describe("CommentsRepository", () => {
 
 The `buildSubject` pattern ensures each `run()` call supplies a fresh instance — every contract `it()` starts with a clean repository.
 
-**File naming convention (post-Plan-8):**
+**File naming convention:**
 
 - Mock implementation: `<x>.repository.mock.ts` (not `mock-<x>.repository.ts`)
 - Mock test: `<x>.repository.mock.test.ts`