feat: replace flat context with tuple-keyed store (#50)

Context keys are now tuples instead of strings, inspired by
React Query's query keys. This prevents context collisions
when multiple instances of the same source type are registered.

Sources write to structured keys like
["aris.google-calendar", "nextEvent", { account: "work" }]
and consumers can query by prefix via context.find().

Co-authored-by: Ona <no-reply@ona.com>

apps/aris-backend/src/session/user-session.test.ts

@@ -1,4 +1,4 @@
-import type { ActionDefinition, Context, FeedSource } from "@aris/core"
+import type { ActionDefinition, ContextEntry, FeedSource } from "@aris/core"
 
 import { LocationSource } from "@aris/source-location"
 import { describe, expect, test } from "bun:test"
@@ -14,7 +14,7 @@ function createStubSource(id: string): FeedSource {
 		async executeAction(): Promise<unknown> {
 			return undefined
 		},
-		async fetchContext(): Promise<Partial<Context> | null> {
+		async fetchContext(): Promise<readonly ContextEntry[] | null> {
 			return null
 		},
 		async fetchItems() {