chore: require a fee receiver at build time, ignore internal docs

The build now fails fast when KYBERSWAP_FEE_RECEIVER is unset instead of
shipping a bundle that sweeps output fees nowhere. Documents the key in
.env.example alongside what it actually costs: 0 bps on direct swaps, 9 bps
on ZAP's embedded swap leg.

Docs are written for the private tree and get copied in by mistake during a
sync, so *.md is ignored by default and publishing one is opt-in.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -10,3 +10,18 @@ indexer-dist
 tsconfig.tsbuildinfo
 *.log
 .vite
+
+# local tool caches/artifacts
+.codegraph/
+graphify-out/
+# Claude Code worktrees — nested checkouts of THIS repo; committing one would
+# bury a duplicate copy of the tree. Project config under .claude/ stays tracked.
+.claude/worktrees/
+
+# Internal notes stay internal. Docs are written for the private tree and get
+# copied in by mistake during a sync; ignoring them means `git add -A` can never
+# publish one. Publishing a doc is opt-in: add it below, deliberately.
+*.md
+!README.md
+# The contract map is on-chain fact, and the source comments cite it by path.
+!docs/up33-contract-map.md