feat(vscode): add T3 Code VS Code extension

[patroza]

Motivation

T3 Code is already amazing on the web, desktop, and mobile. You get a powerful, synchronized coding agent experience everywhere you work.

But there are moments when you're deep inside your editor and you really want the chat right there with you:

• You want to carry your current thread and full conversation context with you without switching tools.
• You want to quickly refer to a particular file or specific line of code directly from the chat.
• You want the chat to just survive — a window reload, an extension host restart, a VS Code update — without losing your thread.

The integration is fully realtime and synchronized. Start a thread in the extension, switch to desktop or mobile, continue the exact same conversation, then jump straight back into VS Code — everything stays perfectly in sync with zero friction or manual steps.

This extension brings the full T3 Code experience directly into VS Code as a first-class citizen.

Features

• A dedicated T3 Code view in the secondary sidebar (or activity bar)
• Native @t3 Chat participant support
• Realtime, fully synchronized threads with your web, desktop, and mobile clients
• Editor context, slash commands, image attachments, approvals, tool rendering, tasks, etc.
• Works with both local T3 Desktop and remote T3 servers

[patroza]

Reviewed the extension. Overall a solid, well-structured contribution that follows the repo's conventions (Effect, contracts, thorough webview→host validation, sanitized markdown, strict CSP with per-load nonce) and has good unit coverage of the pure logic.

One blocking item (activation crash in an empty window), two defense-in-depth hardening items (arbitrary file open via links, broad img-src), plus a few minor notes. Details inline.

[patroza]

Found four blocking integration/correctness issues. The extension tests, build, vp check, and full typecheck pass after installing with a regenerated lockfile, but the committed tree cannot be installed with the repository’s frozen-lockfile policy.