chore: add graft repo context graph integration

Add .claude/ config (settings, graft statusline and hooks helpers,
graft skill), .mcp.json and opencode.json MCP server entries, and
AGENTS.md with graft usage instructions. Add .ignore to re-admit
graft/ to ripgrep search while excluding its cache. Add /graft/ to
.gitignore since the graph is regenerable.

opencode.json

@@ -0,0 +1,12 @@
+{
+  "mcp": {
+    "graft": {
+      "type": "local",
+      "command": [
+        "graft",
+        "mcp"
+      ],
+      "enabled": true
+    }
+  }
+}