fix: make graph output deterministic (stop graphify-out churn) (#1010)

graphify-out regenerates differently on every `graphify update` even
when no source changed, so the committed graph is perpetually dirty and
the post-commit/post-checkout hooks fight every commit. Two independent
nondeterminism sources, each fixed here:

1. Edge direction flips. build.py builds an undirected graph and stores
   direction in _src/_tgt; collapsing two edges onto the same node pair
   is last-write-wins, and unstable edge iteration order flips them
   run-to-run. Fixed by sorting edges by (source, target, relation)
   before the add loop.

2. Clustering churn. The networkx Louvain fallback iterates string-keyed
   sets whose order is randomized per-process by PYTHONHASHSEED, so
   community assignments differ run-to-run even with seed=42. Fixed by
   exporting PYTHONHASHSEED=0 in the generated post-commit and
   post-checkout hook scripts.

With both fixes, `graphify update` is idempotent: rebuilding an
already-converged graphify-out reproduces graph.json and GRAPH_REPORT.md
byte-for-byte.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

graphify/build.py

@@ -160,7 +160,18 @@ def build_from_json(extraction: dict, *, directed: bool = False, root: str | Pat
     # slightly different casing or punctuation than the AST extractor.
     # e.g. "Session_ValidateToken" maps to "session_validatetoken".
     norm_to_id: dict[str, str] = {_normalize_id(nid): nid for nid in node_set}
-    for edge in extraction.get("edges", []):
+    # Iterate edges in a deterministic order. The graph is undirected and stores
+    # direction in _src/_tgt; when two edges collapse onto the same node pair the
+    # last write wins, so an unstable iteration order flips _src/_tgt run-to-run
+    # and makes the serialized graph churn. Sorting fixes the last-write outcome.
+    for edge in sorted(
+        extraction.get("edges", []),
+        key=lambda e: (
+            str(e.get("source", e.get("from", ""))),
+            str(e.get("target", e.get("to", ""))),
+            str(e.get("relation", "")),
+        ),
+    ):
         if "source" not in edge and "from" in edge:
             edge["source"] = edge["from"]
         if "target" not in edge and "to" in edge:

graphify/hooks.py

@@ -48,6 +48,11 @@ _HOOK_SCRIPT = """\
 # Auto-rebuilds the knowledge graph after each commit (code files only, no LLM needed).
 # Installed by: graphify hook install
 
+# Deterministic clustering: networkx louvain iterates string-keyed sets whose
+# order is randomized per-process by PYTHONHASHSEED, so community assignments
+# churn run-to-run. Pinning it makes graphify-out reproducible.
+export PYTHONHASHSEED=0
+
 # Skip during rebase/merge/cherry-pick to avoid blocking --continue with unstaged changes
 GIT_DIR=$(git rev-parse --git-dir 2>/dev/null)
 [ -d "$GIT_DIR/rebase-merge" ] && exit 0
@@ -106,6 +111,11 @@ _CHECKOUT_SCRIPT = """\
 # Auto-rebuilds the knowledge graph (code only) when switching branches.
 # Installed by: graphify hook install
 
+# Deterministic clustering: networkx louvain iterates string-keyed sets whose
+# order is randomized per-process by PYTHONHASHSEED, so community assignments
+# churn run-to-run. Pinning it makes graphify-out reproducible.
+export PYTHONHASHSEED=0
+
 PREV_HEAD=$1
 NEW_HEAD=$2
 BRANCH_SWITCH=$3