feat(grid-agent): isolate conversation memory (#122)

docs/grid-agent-architecture.md

@@ -27,6 +27,7 @@ observable receivers.
 | Offline session work | live supervisor | 1,024 hard / `reconnect.offline_work_capacity` | read-only queue; mutations rejected while not ready |
 | Body / message | typed boundary owners | 8 MiB / 64 KiB hard ceilings, with lower configured limits | rejected before enqueue |
 | Conversation / tool calls | request owner | 256 messages / 64 calls, with lower configured limits | rejected before request |
+| Per-avatar conversation memory | `ConversationStore` mutex | 4,096 sessions / 64 MiB hard, lower `conversation` limits | monotonic expiry, deterministic compaction/LRU eviction |
 | LLM request slots | shared `LlmClient` semaphore | 256 hard / configured concurrent requests | async acquire or cancellation |
 | Reasoning/tool session | `ToolLoop` caller | 32 turns / 256 calls hard, with lower configured limits | total timeout, cancellation, or supersession |
 | Policy tools / approvals / schedules | `PolicyGateway` mutex | 64 tools / 4,096 approval records / 1,024 scheduler grants hard | deny before opaque authorization |
@@ -84,6 +85,10 @@ cleanup, fencing old events and late LLM/tool results. See
   are refused, response bodies are bounded while streaming, bearer secrets are
   redacted, and provider/model discovery does not exist. Proposed calls cross
   `ToolExecutor` only after registered-name and schema validation.
+- Conversation context is keyed by immutable avatar UUID and either public chat
+  or direct IM. Group channels are not representable. The LLM projection can
+  retrieve only one exact key, and recovered/untrusted summaries remain user-role
+  prompt data rather than system authority.
 - Signals and console output belong to the binary. The reusable core relies on
   no terminal, Unix socket, Unix signal, separator, or fixed platform path.
 
@@ -116,3 +121,5 @@ The origin/capability matrix and opaque mutation boundary are documented in
 [`grid-agent-policy.md`](grid-agent-policy.md).
 The live lifecycle and generation contract is documented in
 [`grid-agent-session.md`](grid-agent-session.md).
+The conversation isolation and persistence contract is documented in
+[`grid-agent-conversation.md`](grid-agent-conversation.md).