docs(agentstack): note dual-runtime support (Bun + Node.js + Workers + browser)

Matches the revised Shared AppKit PRD's Runtime Compatibility Strategy.
AgentStack is runtime-neutral (no Bun/Node-only APIs; time is injected).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ralyodio

docs/agentstack.md

@@ -50,6 +50,12 @@ payment, escrow, and reputation stay linked. Terminal statuses (`complete`, `fai
   `updateTaskStatus`, `delegate`, `revokeDelegation`, `listTasks`, `snapshot`, `on`
 - `agentStackPlugin` — a validated LogicSRC `PluginDefinition`
 
+## Runtime
+
+Runtime-neutral by design — pure TypeScript, no Bun- or Node-only APIs (time is injected via
+the `AgentStack` constructor). Runs on Bun, Node.js, Cloudflare Workers, and the browser,
+matching the Profullstack dual-runtime standard (Bun + Node.js both first-class).
+
 ## Plugin manifest
 
 ```jsonc

packages/agentstack/README.md

@@ -51,6 +51,12 @@ sh1pt.users.id -> did:coinpay:user:123
 ugig.users.id  -> did:coinpay:user:123
 ```
 
+## Runtime
+
+Runtime-neutral by design — pure TypeScript with no Bun- or Node-only APIs (time is injected
+via the `AgentStack` constructor). Runs on **Bun, Node.js, Cloudflare Workers, and the
+browser**, matching the Profullstack dual-runtime standard.
+
 ## Environment
 
 ```txt