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/releases/index.md

@@ -0,0 +1,10 @@
+# Release Index
+
+## 목적
+- 이 문서는 release 단위의 전역 인덱스다.
+- 각 release는 하나 이상의 release-candidate batch를 묶고, profile(`docs-only` | `tooling` | `app-service`)에 맞는 배포와 후속 환류를 추적한다.
+
+## Release Register
+
+| REL ID | Profile | Status | Included Batch | Discovery/REQ Input | Link |
+| --- | --- | --- | --- | --- | --- |