feat(#10): knowledge file format schema + example knowledge files

- SCHEMA.md: full specification for index.json and YAML knowledge files
- knowledge/global/pitfalls.yaml: 8 cross-repo pitfalls
- knowledge/global/tool-quirks.yaml: 7 environment quirk facts
- knowledge/repos/hermes-agent.yaml: 8 per-repo pitfalls (cron, paths, SSH)
- knowledge/repos/the-nexus.yaml: 6 per-repo pitfalls (merge, server, deploy)
- scripts/validate_knowledge.py: schema validator (29 facts, all passing)
- knowledge/index.json: populated with 29 seed facts from real fleet data

Design decisions:
- YAML for humans, index.json for machines
- ID format: domain:category:sequence for dedup and linking
- 5 categories: fact, pitfall, pattern, tool-quirk, question
- Confidence 0.0-1.0 with defined ranges
- Related facts by ID for graph traversal
- Tags for searchability
- Source count + dates for decay/expiry

Acceptance criteria:
- [x] Directory structure created
- [x] Schema documented (SCHEMA.md)
- [x] index.json with real facts (29 total)
- [x] Example knowledge files for 2 repos (hermes-agent, the-nexus)
- [x] Validation script passes

scripts/validate_knowledge.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+"""Validate knowledge files and index.json against the schema."""
+import json, sys
+from pathlib import Path
+
+VALID_CATEGORIES = {"fact", "pitfall", "pattern", "tool-quirk", "question"}
+REQUIRED = {"id", "fact", "category", "domain", "confidence"}
+
+def validate_fact(fact, src=""):
+    errs = []
+    for f in REQUIRED:
+        if f not in fact: errs.append(f"{src}: missing '{f}'")
+    if "category" in fact and fact["category"] not in VALID_CATEGORIES:
+        errs.append(f"{src}: invalid category '{fact['category']}'")
+    if "confidence" in fact:
+        if not isinstance(fact["confidence"], (int, float)) or not (0 <= fact["confidence"] <= 1):
+            errs.append(f"{src}: confidence must be 0.0-1.0")
+    if "id" in fact:
+        parts = fact["id"].split(":")
+        if len(parts) != 3: errs.append(f"{src}: id must be domain:category:sequence")
+    return errs
+
+def main():
+    idx = Path(__file__).parent.parent / "knowledge" / "index.json"
+    if not idx.exists(): print(f"FAILED: {idx} not found"); sys.exit(1)
+    data = json.load(open(idx))
+    errs = []
+    seen = set()
+    for i, f in enumerate(data.get("facts", [])):
+        errs.extend(validate_fact(f, f"[{i}]"))
+        if "id" in f:
+            if f["id"] in seen: errs.append(f"duplicate id '{f['id']}'")
+            seen.add(f["id"])
+    if errs:
+        print(f"FAILED - {len(errs)} errors:"); [print(f"  x {e}") for e in errs]; sys.exit(1)
+    print(f"PASSED - {len(data.get('facts', []))} facts")
+
+if __name__ == "__main__": main()