feat: add initial

Signed-off-by: kjuulh <contact@kjuulh.io>

todos/SQ-020-cluster-membership.md

@@ -0,0 +1,42 @@
+# SQ-020: Cluster Membership (Gossip)
+
+**Status:** `[ ] TODO`
+**Blocked by:** SQ-009, SQ-019
+**Priority:** Medium
+
+## Description
+
+Nodes discover each other via seed list and maintain a membership list through periodic heartbeats.
+
+## Files to Create/Modify
+
+- `crates/sq-cluster/src/lib.rs` - module exports
+- `crates/sq-cluster/src/membership.rs` - seed list, join, heartbeat, failure detection
+- `crates/sq-server/src/grpc/cluster.rs` - ClusterService Join/Heartbeat RPC impl
+- `crates/sq-server/src/cli/serve.rs` - add --seeds CLI flag
+
+## Configuration
+
+```
+SQ_SEEDS=node1:6060,node2:6060   # Seed node addresses
+SQ_NODE_ID=node-1                 # Unique node ID
+SQ_HEARTBEAT_INTERVAL_MS=5000    # Heartbeat every 5s
+SQ_FAILURE_THRESHOLD=3           # Missed heartbeats before suspected
+```
+
+## Membership State Machine
+
+```
+Unknown -> Alive (on Join response or Heartbeat)
+Alive -> Suspected (missed 3 heartbeats)
+Suspected -> Dead (suspected for 30 seconds)
+Dead -> Alive (on successful re-Join)
+```
+
+## Acceptance Criteria
+
+- [ ] Start 3 nodes with seed list, all discover each other
+- [ ] Status RPC shows all 3 nodes as "alive"
+- [ ] Stop one node, others detect it as "suspected" then "dead"
+- [ ] Restart dead node, it re-joins and becomes "alive"
+- [ ] Node with no seeds starts as single-node cluster