Merge feature/sigit-vscode-v1 into main: siGit Code 1.0.0

Initial release of the ACP client extension. Highlights: - ACP transport, client, and chat webview with branded header - pnpm + signed CI; mock-agent smoke test (round-trip verified) - Extension icon (media/icon.png) and Marketplace metadata - Author/identity hardened: paydii, GPG-Verified Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_014EUvyohLND97Xxew23jN2d