Lower minimum VS Code to 1.105, remove Coder Chat sidebar, harden deeplink params

[EhabY]

Problems

1. The minimum VS Code bump to 1.106.0 (shipped in v1.14.0) blocks the latest Cursor stable. Cursor 3.5 reports VS Code 1.105.1 to extensions, so users on Cursor can no longer install our extension. The floor needs to come back down to 1.105.
2. Product is retiring the experimental Coder Chat secondary sidebar. The agents experiment gate, the chat webview, the chatId deeplink handoff, and the @repo/chat iframe shim package should all be removed entirely (not just flag-gated off).
3. The vscode:// deeplink handler already silently drops unknown query params, but nothing pins that contract. With chat gone, older Coder servers will keep sending chatId and newer servers may add params this client has not implemented yet. A regression test should lock the permissive behaviour in.

Desired outcome

• engines.vscode lowered to ^1.105.0; @types/vscode to 1.105.0; the CI integration matrix pinned at 1.105.1 (the patch Cursor ships).
• All Coder Chat code, contributions, context gates, and tests removed.
• New unit tests in test/unit/uri/uriHandler.test.ts assert that unknown query params (older chatId, future server params) do not raise an error modal and do not change the open behaviour.