Docs: add README with setup, config, and curl testing instructions

Co-Authored-By: Lars Baunwall <larslb@thinkability.dk>

README.md

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+# VS Code Copilot Bridge (Desktop, Inference-only)
+
+Local OpenAI-compatible HTTP facade to GitHub Copilot Chat via the VS Code Chat provider.
+
+- Endpoints (local-only, default bind 127.0.0.1):
+  - POST /v1/chat/completions (SSE streaming; use "stream": false for non-streaming)
+  - GET /v1/models (synthetic listing: gpt-4o-copilot)
+  - GET /healthz (ok/unavailable + vscode.version)
+
+- Copilot pipe: vscode.chat.requestChatAccess('copilot') → startSession().sendRequest({ prompt })
+
+- Prompt normalization: last system message + last N user/assistant turns (default 3) rendered as:
+  [SYSTEM]
+  …
+  [DIALOG]
+  user: …
+  assistant: …
+
+- Security: loopback-only binding by default; optional Authorization: Bearer <token>.
+
+## Install, Build, and Run
+
+Prerequisites:
+- VS Code Desktop (stable), GitHub Copilot signed in
+- Node.js 18+ (recommended)
+- npm
+
+Steps:
+1) Install deps and compile:
+   npm install
+   npm run compile
+
+2) Launch in VS Code (recommended for debugging):
+   - Open this folder in VS Code
+   - Press F5 to run the extension in a new Extension Development Host
+
+3) Enable the bridge:
+   - Command Palette → “Copilot Bridge: Enable”
+   - Or set in settings: bridge.enabled = true
+   - “Copilot Bridge: Status” shows the bound address/port and token requirement
+
+Optional: Packaging a VSIX
+- You can package with vsce (not included):
+  npm i -g @vscode/vsce
+  vsce package
+- Then install the generated .vsix via “Extensions: Install from VSIX…”
+
+## Configuration (bridge.*)
+
+- bridge.enabled (boolean; default false): auto-start on VS Code startup
+- bridge.host (string; default "127.0.0.1"): bind address (keep on loopback)
+- bridge.port (number; default 0): 0 = ephemeral port
+- bridge.token (string; default ""): optional bearer token; empty disables auth
+- bridge.historyWindow (number; default 3): number of user/assistant turns to keep
+- bridge.maxConcurrent (number; default 1): max concurrent chat requests; excess → 429
+- bridge.verbose (boolean; default false): verbose logs to “Copilot Bridge” output channel
+
+## Manual Testing (curl)
+
+Replace <port> with the port shown in “Copilot Bridge: Status”.
+
+Health:
+curl http://127.0.0.1:<port>/healthz
+
+Models:
+curl http://127.0.0.1:<port>/v1/models
+
+Streaming completion:
+curl -N -H "Content-Type: application/json" \
+  -d '{"model":"gpt-4o-copilot","stream":true,"messages":[{"role":"user","content":"hello"}]}' \
+  http://127.0.0.1:<port>/v1/chat/completions
+
+Non-stream:
+curl -H "Content-Type: application/json" \
+  -d '{"model":"gpt-4o-copilot","stream":false,"messages":[{"role":"user","content":"hello"}]}' \
+  http://127.0.0.1:<port>/v1/chat/completions
+
+Auth (when bridge.token is set):
+- Missing/incorrect Authorization: Bearer <token> → 401
+
+Copilot unavailable:
+- Sign out of GitHub Copilot; /healthz shows unavailable; POST returns 503 envelope
+
+Concurrency:
+- Fire 2+ concurrent requests; above bridge.maxConcurrent → 429 rate_limit_error
+
+## Notes
+
+- Desktop-only, in-process. No VS Code Server dependency.
+- Single-user, local loopback. Do not expose to remote interfaces.
+- Non-goals: tools/function calling emulation, workspace file I/O, multi-tenant proxying.
+
+## Development
+
+- Build: npm run compile
+- Watch: npm run watch
+- Main: src/extension.ts
+- Local type shim for Chat API: src/vscode-chat-shim.d.ts (for TypeScript on stable @types/vscode)