feat(memory): add opt-in scoped memory tools

docs/adr/0001-opt-in-plugin-storage-memory.md

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+# 0001. Store explicit memory through the V2 plugin storage API
+
+Date: 2026-10-08
+Status: proposed
+
+## Context
+
+The toolkit needs persistent memory without assuming OpenCode's on-disk storage
+layout, silently collecting session data, or mixing sessions and projects.
+OpenCode 2.0.24 exposes plugin-scoped storage and supplies the active tool
+session ID and session project/location through its public plugin API.
+
+## Decision
+
+Use `ctx.storage` for explicitly invoked memory operations. Disable memory unless
+the plugin's `memory.enabled` option is true; default writes to a key namespace
+derived from project identity and session ID. Permit canonical-project scope
+only when `memory.project` is separately enabled. Return content only through an
+explicit retrieval/list tool call, apply a configurable expiry, and do not
+capture prompts or automatically inject stored content into model context.
+
+## Consequences
+
+- The implementation avoids dependence on private OpenCode database paths and
+  makes opt-in boundaries visible in project configuration.
+- Project/session key namespaces limit accidental data mixing, while project
+  sharing deliberately spans sessions and worktrees with the same canonical
+  project identity.
+- Storage encryption, user-profile isolation, and persistence across a live
+  OpenCode restart rely on the host implementation and remain unverified.
+- Lazy expiration does not immediately reclaim inaccessible expired records;
+  there is no background cleanup or automatic learning workflow.