Initial scaffold: tooling, plan, task briefs

- pnpm 10 workspace + TypeScript strict + ESLint flat + Prettier
- CLAUDE.md as canonical brief
- docs/tasks/ — 22 task briefs broken out by phase for sub-sessions
- docs/decisions/ — ADR scaffold

Implements task 01 (repo-init).

.editorconfig

@@ -0,0 +1,12 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+indent_size = 2
+trim_trailing_whitespace = true
+
+[*.md]
+trim_trailing_whitespace = false