[docs] Draft playground workspace PRD and initial RFC

docs/design/playground-workspace/README.md

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+# Playground workspace discussion
+
+> Draft for founder and frontend review. No implementation or scope approval is implied.
+
+Date: 2026-09-11.
+
+[Explore the interactive mockup](https://agenta-playground-workspace-concept.mahmoud-637.workers.dev) or read its [walkthrough and source notes](mockup/README.md).
+
+The proposal turns the agent playground into one or two panes with movable tabs. A conversation, file, explorer, or application can occupy either pane.
+
+Read [the product requirements document](prd.md) for the intended experience, then [the initial request for comments](rfc.md) for the architecture and tradeoffs. Both are deliberately high level. There is no implementation schedule or final interface contract.
+
+Related: [PR #6529: workspace-backed internal HTML apps](https://github.com/Agenta-AI/agenta/pull/6529). That proposal covers executing HTML and granting file access. This proposal covers presenting those apps alongside conversations and files.
+
+Terms used in these documents:
+
+- A resource is an underlying session, file, application, or remote computer.
+- A tab is an open view of a resource.
+- A pane holds ordered tabs and displays its selected tab.
+- A workspace stores the panes, open tabs, and keyboard focus for an agent's playground.
+- A runtime is a live client object, such as a chat connection, whose lifetime can outlast its React view.
+- A sandbox is the isolated environment where an agent runs tools or applications.
+
+Status: initial discussion drafts. The founder requested the split-pane direction and the candidate content types. The proposed first scope, lifecycle policies, and library preference are agent suggestions awaiting review.

docs/design/playground-workspace/mockup/README.md

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+# Interactive playground concept
+
+[Open the Cloudflare preview](https://agenta-playground-workspace-concept.mahmoud-637.workers.dev).
+
+This desktop mockup illustrates the [PRD](../prd.md) and [RFC](../rfc.md). It uses local sample data and makes no agent, mounts, or remote-desktop API calls. Website and computer tabs are explicit placeholders. The Kanban example demonstrates the relationship to [#6529](https://github.com/Agenta-AI/agenta/pull/6529); it does not implement that proposal's HTML execution or permission bridge.
+
+## Try the idea
+
+1. Use the three scenario buttons to see a conversation beside a file, an app, or another conversation.
+2. Type a draft, then drag its tab into the other pane or use the pane's move button. The draft stays with the session. Dropping before another tab also changes order.
+3. Resize the divider with the pointer or focus it and press an arrow key. Switch to one pane to bring all open tabs together.
+4. Click a file card in the conversation. An already-open file receives focus; a new one opens in the other pane.
+5. Open the app scenario and move a board card. Select Files and expand `board.json` to see the same example data.
+6. Use the plus button to open another kind of tab, and the close button to close the selected tab. Reset restores the initial example.
+
+The layout, conversation drafts, example messages, and board data persist in this browser's local storage. The sidebar is visual context. On small screens the split workspace scrolls horizontally; the proposed mobile pane switcher is not implemented here.
+
+## Component reuse
+
+The mockup imports the repository's actual Button, Tabs, and SplitPane source from `web/packages/agenta-ui/src/components/ui/`. It also imports Agenta's generated theme variables, control scale, and shadcn token mapping. No copies of those components are maintained here.
+
+Relative source imports are deliberate for this standalone concept. They avoid loading the application's provider stack and require running the mockup inside a complete repository checkout. This is not the proposed production package structure.
+
+The content views and local interaction state are specific to the mockup. Native drag-and-drop illustrates tab placement; the move button supplies a keyboard-accessible alternative. Production runtime ownership, streaming, permissions, and complete accessibility remain subjects of the RFC.
+
+## Build and deploy
+
+From this directory, with Node.js 20 or newer:
+
+```sh
+npm ci --include=dev
+npx tsc --noEmit
+npm run build
+```
+
+The standalone dependency lock is local to this directory. A root application install is not required. Vite deduplicates component dependencies against this host; TypeScript uses local dependency paths for the same reason.
+
+With Cloudflare credentials supplied securely through the environment:
+
+```sh
+npx wrangler@4.72.0 deploy
+```
+
+The Worker serves static assets only. No credentials belong in the source or built assets.
+
+## Verification
+
+Built the static bundle and passed TypeScript checking. Tested the hosted URL with headless Chromium: tab movement by button and drag, draft preservation across move/collapse/reload, keyboard divider resizing, adding and closing tabs, and board data reflected in Files. Inspected screenshots in light and dark themes. No browser JavaScript errors appeared in the exercised flows.
+
+This verifies the concept's local interactions, not production chat performance or backend behavior.
+
+![Conversation beside a document](preview.png)
+
+![Conversation beside the example app](preview-app.png)

docs/design/playground-workspace/mockup/index.html

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+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>Agenta · Playground workspace concept</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="/src/main.tsx"></script>
+  </body>
+</html>