Bootstrap StagePilot baseline for slot domain migration

Initialize baseline seed, four baseline docs (project-structure,
runtime-flows, interface-contract, data-model), and the active
discovery/srs/batches/releases indexes. Content derived from the
trex-orbit slot pure-domain migration plan (trex-core as a new base,
pure domain only).

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

docs/batches/index.md

@@ -0,0 +1,10 @@
+# Batch Index
+
+## 목적
+- 이 문서는 batch delivery 단위의 전역 인덱스다.
+- 각 batch는 `docs/batches/<BAT_ID>/` 아래에서 profile(`standard` | `batch-lite`)에 맞는 산출물을 관리한다.
+
+## Batch Register
+
+| BAT ID | Profile | Status | Included REQ | Discovery | Folder |
+| --- | --- | --- | --- | --- | --- |