fix(broker): shareContext stable upsert key + createStream atomic upsert

- shareContext: adds optional memberId param; when provided, upserts on
  (meshId, memberId) instead of (meshId, presenceId) — prevents stale
  context rows accumulating on every reconnect. Falls back to presenceId
  for legacy/anonymous connections. Also refreshes presenceId on update
  so it stays current.
- schema: adds member_id column + unique index context_mesh_member_idx
  on mesh.context table; new migration 0013_context-stable-member-key.sql.
- index.ts call site updated to pass conn.memberId as the stable key.
- createStream: replaces SELECT-then-INSERT TOCTOU race with atomic
  INSERT ... ON CONFLICT DO NOTHING RETURNING, followed by SELECT on miss.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/broker/src/broker.ts

@@ -955,8 +955,13 @@ export async function grantFileKey(
 // --- Context sharing ---
 
 /**
- * Upsert a context snapshot for a peer. Each (meshId, presenceId) pair
- * has at most one context row — repeated calls update it in place.
+ * Upsert a context snapshot for a peer. When `memberId` is provided the
+ * row is keyed on (meshId, memberId) — a stable identifier that survives
+ * reconnects. This prevents stale rows from accumulating every time a
+ * session reconnects with a fresh ephemeral presenceId.
+ *
+ * Falls back to (meshId, presenceId) lookup when memberId is absent
+ * (e.g. legacy callers or anonymous connections).
  */
 export async function shareContext(
   meshId: string,
@@ -966,24 +971,27 @@ export async function shareContext(
   filesRead?: string[],
   keyFindings?: string[],
   tags?: string[],
+  memberId?: string,
 ): Promise<string> {
   const now = new Date();
-  // Try to find existing context for this presence in this mesh.
+
+  // Build the WHERE clause: prefer stable memberId, fall back to presenceId.
+  const lookupWhere = memberId
+    ? and(eq(meshContext.meshId, meshId), eq(meshContext.memberId, memberId))
+    : and(eq(meshContext.meshId, meshId), eq(meshContext.presenceId, presenceId));
+
   const [existing] = await db
     .select({ id: meshContext.id })
     .from(meshContext)
-    .where(
-      and(
-        eq(meshContext.meshId, meshId),
-        eq(meshContext.presenceId, presenceId),
-      ),
-    )
+    .where(lookupWhere)
     .limit(1);
 
   if (existing) {
     await db
       .update(meshContext)
       .set({
+        // Keep presenceId current so it reflects the latest connection.
+        presenceId,
         peerName: peerName ?? null,
         summary,
         filesRead: filesRead ?? [],
@@ -999,6 +1007,7 @@ export async function shareContext(
     .insert(meshContext)
     .values({
       meshId,
+      memberId: memberId ?? null,
       presenceId,
       peerName: peerName ?? null,
       summary,
@@ -1248,16 +1257,22 @@ export async function createStream(
   name: string,
   createdByName: string,
 ): Promise<string> {
-  const existing = await db
+  // Atomic upsert: INSERT ... ON CONFLICT DO NOTHING to avoid TOCTOU race
+  // when two callers concurrently attempt to create the same stream.
+  const [inserted] = await db
+    .insert(meshStream)
+    .values({ meshId, name, createdByName })
+    .onConflictDoNothing()
+    .returning({ id: meshStream.id });
+
+  if (inserted) return inserted.id;
+
+  // Row already existed — fetch the id.
+  const [existing] = await db
     .select({ id: meshStream.id })
     .from(meshStream)
     .where(and(eq(meshStream.meshId, meshId), eq(meshStream.name, name)));
-  if (existing.length > 0) return existing[0]!.id;
-  const [row] = await db
-    .insert(meshStream)
-    .values({ meshId, name, createdByName })
-    .returning({ id: meshStream.id });
-  return row!.id;
+  return existing!.id;
 }
 
 /**

packages/db/migrations/0013_context-stable-member-key.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "mesh"."context" ADD COLUMN "member_id" text;--> statement-breakpoint
+ALTER TABLE "mesh"."context" ADD CONSTRAINT "context_member_id_member_id_fk" FOREIGN KEY ("member_id") REFERENCES "mesh"."member"("id") ON DELETE cascade ON UPDATE cascade;--> statement-breakpoint
+CREATE UNIQUE INDEX "context_mesh_member_idx" ON "mesh"."context" ("mesh_id","member_id");

packages/db/src/schema/mesh.ts

@@ -353,24 +353,37 @@ export const meshFileKeyRelations = relations(meshFileKey, ({ one }) => ({
 }));
 
 /**
- * Per-peer context snapshot. Each peer (presence) has at most one context
+ * Per-peer context snapshot. Each peer (member) has at most one context
  * entry per mesh, upserted on each share_context call. Allows peers to
  * discover what others are working on, which files they've read, and
  * key findings — without sending a direct message.
+ *
+ * `memberId` is the stable upsert key (survives reconnects). `presenceId`
+ * is kept for backwards-compat but is nullable — new rows should always
+ * populate `memberId`. The unique index on (meshId, memberId) prevents
+ * stale rows from accumulating when a session reconnects with a new
+ * ephemeral presenceId.
  */
-export const meshContext = meshSchema.table("context", {
-  id: text().primaryKey().notNull().$defaultFn(generateId),
-  meshId: text()
-    .references(() => mesh.id, { onDelete: "cascade", onUpdate: "cascade" })
-    .notNull(),
-  presenceId: text().references(() => presence.id, { onDelete: "cascade" }),
-  peerName: text(),
-  summary: text().notNull(),
-  filesRead: text().array().default([]),
-  keyFindings: text().array().default([]),
-  tags: text().array().default([]),
-  updatedAt: timestamp().defaultNow().notNull(),
-});
+export const meshContext = meshSchema.table(
+  "context",
+  {
+    id: text().primaryKey().notNull().$defaultFn(generateId),
+    meshId: text()
+      .references(() => mesh.id, { onDelete: "cascade", onUpdate: "cascade" })
+      .notNull(),
+    memberId: text().references(() => meshMember.id, { onDelete: "cascade", onUpdate: "cascade" }),
+    presenceId: text().references(() => presence.id, { onDelete: "cascade" }),
+    peerName: text(),
+    summary: text().notNull(),
+    filesRead: text().array().default([]),
+    keyFindings: text().array().default([]),
+    tags: text().array().default([]),
+    updatedAt: timestamp().defaultNow().notNull(),
+  },
+  (table) => [
+    uniqueIndex("context_mesh_member_idx").on(table.meshId, table.memberId),
+  ],
+);
 
 /**
  * Mesh-scoped task board. Peers can create tasks, claim them, and mark