add storage foundation contracts

docs/009-storage-foundation.md

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+# 009 Storage foundation
+
+## Context
+- v1 storage slice needs workspace, chat and file flows before durable DB
+- trusted caller passes `user_id`, and one workspace belongs to one user
+- chat content must live outside sandbox lifecycle and survive sandbox restart
+
+## Decision
+- metadata repositories are in-memory for the first storage slice
+- `Workspace`, `Chat` and `ChatFile` are first-class domain entities
+- filesystem access stays behind storage ports in outer layers
+- sandbox later integrates through chat metadata and storage ports, not raw path math in usecases
+
+## Consequences
+- metadata is lost on restart in this phase
+- storage usecases and HTTP API can be built before durable persistence
+- later durable metadata can replace in-memory adapters behind the same ports

docs/010-chat-history-policy.md

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+# 010 Chat history policy
+
+## Context
+- v1 keeps chat history in filesystem, not in central DB
+- chat metadata must not depend on parsing history content
+- each chat already maps to an isolated working directory
+
+## Decision
+- each chat owns one `history.md` inside its chat directory
+- `history.md` is created with chat layout initialization
+- chat metadata stores identity and lifecycle fields separately from history content
+- history read and write stay behind storage ports in outer layers
+
+## Consequences
+- history survives sandbox restart with chat storage
+- metadata and content evolve independently
+- later migration to another history backend can keep the same chat identity model