feat(pipeline): skip extraction when the source has not changed (#105) (#126)

Co-authored-by: Mathias <mthbqv@gmail.com>

README.md

@@ -100,8 +100,21 @@ review gate.
 Re-ingesting unchanged content is **byte-identical** — no diff, so brain-sync
 does not commit noise that hides real edits.
 
-Known limitation: because the on-disk frontmatter wins, `last_updated` keeps its
-first-ingest date on a revised page.
+Two fields are **generator-owned** and do get refreshed on a revise:
+`last_updated` and `source_hash`. Everything else on disk wins. The boundary is
+an explicit allowlist, so a field nobody has thought about stays human-owned.
+
+## Re-ingesting an unchanged source is skipped
+
+Ingest stores `source_hash` — sha256 of the ingested **body**, frontmatter
+excluded — on the source page. A later ingest of the same source compares it
+first and skips extraction entirely when it matches, reporting the skip rather
+than doing it silently.
+
+Body-only is deliberate: a re-crawl that stamps a new fetch date has not changed
+the source, and treating that as drift would re-extract the corpus every time a
+crawler ran. A missing or unreadable hash always means "extract" — a missed skip
+costs one extraction, a wrong skip loses an update.
 
 ## Phase 1 tools (available now)