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.

.gitignore

@@ -16,3 +16,6 @@ compile_commands.json
 *.user
 .DS_Store
 .cache/
+
+# graft's local graph cache — regenerable, not committed (run `graft build`).
+/graft/