fix(ingest): exclude QUESTION_PLAN.md from corpus ingestion

scripts/ingest.py already excluded SOURCES.md and README.md as
version-controlled curation artifacts. QUESTION_PLAN.md (new at
3241b7c) is the same class of artifact — it belongs next to
SOURCES.md as authoring guidance, not in the RAG corpus.

Caught during Week 1 step 4 K8s ingestion: the first make ingest-k8s
run indexed 29 unique sources instead of the expected 28, and the
store contents showed QUESTION_PLAN.md as an ingested source. This
would have surfaced QUESTION_PLAN.md chunks in retrieval results at
query time — the k8s_docs/QUESTION_PLAN.md filename would appear in
citations, which is wrong shape for the corpus.

Post-fix re-ingest: 28 unique sources, 2447 chunks. Matches the
locked SOURCES.md breakdown.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/ingest.py

@@ -35,8 +35,9 @@ def ingest(
         sys.exit(1)
 
     # Exclude curation metadata files that live alongside corpus content.
-    # SOURCES.md is a version-controlled curation artifact, not corpus content.
-    _EXCLUDED = {"SOURCES.md", "README.md"}
+    # SOURCES.md and QUESTION_PLAN.md are version-controlled curation
+    # artifacts, not corpus content.
+    _EXCLUDED = {"SOURCES.md", "QUESTION_PLAN.md", "README.md"}
     md_files = sorted(f for f in doc_path.glob("*.md") if f.name not in _EXCLUDED)
     if not md_files:
         print(f"Error: no markdown files found in {doc_dir}")