docs(dox): build out the DOX AGENTS.md hierarchy

Root Child DOX Index assembled, plus a per-module AGENTS.md across the tree
(core/*, infrastructure/*, apps/*, testing/*, and docs/examples/frontend/etc),
each following the DOX section shape: Purpose, Ownership, Local Contracts,
Work Guidance, Verification, Child DOX Index.

infrastructure/artifacts-cas/AGENTS.md

@@ -0,0 +1,33 @@
+# infrastructure/artifacts-cas/
+
+## Purpose
+
+Content-addressable artifact store backed by append-only segment files on disk and a SQLite index. Implements `core:artifacts-store` (`ArtifactStore`). Handles content hashing, segment layout, compaction, eviction, and crash-recovery tail scanning.
+
+## Ownership
+
+Adapter for local filesystem artifact storage. No dependency on other `infrastructure:*` modules at runtime; `infrastructure:persistence` is a test dependency only.
+
+## Local Contracts
+
+- Implements `ArtifactStore` from `core:artifacts-store`.
+- Segment files are append-only; `SegmentWriter`/`SegmentReader` own the on-disk format (`SegmentLayout`).
+- `SqliteArtifactIndex` maintains the content-hash → segment offset index; the SQLite file lives alongside the segments in the configured root dir.
+- `TailScanner` recovers the index after a crash by scanning the tail of the most recent segment.
+- `LivenessScanner` and `Evictor` handle compaction and space reclamation.
+- `CasConfig` is the only configuration surface; pass via `InfrastructureModule.createArtifactStore()`.
+- Environment observations (file existence, segment bytes) happen at call time and must not be re-read during replay — callers must record artifact content as events if replay correctness requires it (invariant #9).
+
+## Work Guidance
+
+Standard adapter rules apply (see parent `AGENTS.md`). No domain logic beyond storage mechanics belongs here.
+
+## Verification
+
+```
+./gradlew :infrastructure:artifacts-cas:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).