Feature request: local web interface (like `opencode web`)

## Motivation

OpenCode ([opencode.ai](https://opencode.ai)) recently shipped `opencode web` — a command that starts a **local HTTP server and serves a browser-based UI** for the AI coding agent. This is a killer feature that Copilot CLI currently lacks.

Copilot CLI is terminal-only (TUI), which limits the experience to one dimension.

## What `opencode web` does

```
$ opencode web
▄
█▀▀█ █▀▀█ █▀▀█ █▀▀▄ █▀▀▀ █▀▀█ █▀▀█ █▀▀█
█ █ █ █ █▀▀▀ █ █ █ █ █ █ █ █▀▀▀
▀▀▀▀ █▀▀▀ ▀▀▀▀ ▀▀▀▀ ▀▀▀▀ ▀▀▀▀ ▀▀▀▀ ▀▀▀▀

Web interface: http://127.0.0.1:4096/
```

Key capabilities:
- **Local HTTP server** bound to `127.0.0.1` with a configurable port (`--port`)
- **Full browser UI** — chat, file tree, diff viewer, session management
- **TUI attach** — `opencode attach http://localhost:4096` lets you connect a terminal session to the same server, sharing state/sessions
- **Auth** — optional HTTP basic auth via `OPENCODE_SERVER_PASSWORD`
- **mDNS discovery** — `--mdns` for LAN access
- **CORS support** — `--cors` for custom domains

## Proposed feature: `copilot web`

```bash
$ copilot web
# Starts a local HTTP server and opens the browser
# equivalent to: copilot --acp + web UI frontend
```

### Benefits over terminal-only

| Feature | Terminal (TUI) | Web interface |
|---|---|---|
| Syntax-highlighted diffs | Basic | Rich, side-by-side |
| File tree navigation | Manual `@` mentions | Clickable tree |
| Multi-session management | `/resume` | Session tabs |
| Code review UX | Scroll through text | Inline comments, annotations |
| Mobile/tablet use | Not practical | Fully usable |
| Accessibility | Screen-reader flag | Native browser a11y |
| Sharing | `--share-gist` | URL sharing, screenshots |

### Implementation notes

- Copilot CLI already has the `--acp` (Agent Client Protocol) server mode — the web UI could be a thin client on top of that
- Could be implemented as a built-in plugin or separate package
- The `--remote` flag already enables GitHub web control — but that is cloud-dependent; a local web server is simpler, faster, and works offline

## References

- OpenCode web docs: https://opencode.ai/docs/cli#web
- OpenCode repo: https://github.com/anomalyco/opencode

Start by reading the existing --acp server mode and --remote entry points, then determine how a local web server and browser UI would fit the CLI. Done would mean a documented copilot web command that serves the proposed local interface with the described session and terminal integration.

Written by the indexing model from the issue text.