fix(fb-40): address PR #132 round 1 review

Module doc still pinned "rag-v1" — update to reflect dispatched
template via system_prompt_for (rag-v1 legacy / rag-v2 default).

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

crates/kebab-rag/src/pipeline.rs

@@ -10,7 +10,9 @@
 //! 3. Pack context — fetch full chunk text via `DocumentStore` and pack
 //!    until the `max_context_tokens` budget is exhausted (estimated at
 //!    ~4 chars / token, matching the kb-chunk convention).
-//! 4. Render the `rag-v1` prompt (system + user) verbatim per design.
+//! 4. Render the configured `prompt_template_version` prompt (system +
+//!    user) verbatim per design — `rag-v1` legacy or `rag-v2` (default,
+//!    fb-40) selected via `system_prompt_for`.
 //! 5. Generate via `LanguageModel::generate_stream`. The token loop runs
 //!    on the calling thread; `opts.stream_sink` (if any) emits
 //!    `StreamEvent::RetrievalDone` once after retrieve+stale-stamp,