v0.16: Antigravity & GitHub Copilot providers

Two new providers join the roster, bringing DevChain to five coordinated agent CLIs.

Highlights

• New provider: Google Antigravity (agy) — with backend, MCP, UI, mobile, infra, and docs parity.
• New provider: GitHub Copilot CLI (copilot) — with deterministic session binding, UI, mobile, and session-start lifecycle hooks.
• Gemini CLI provider retired — templates now redirect to Antigravity (agy).
• E2EE hardening — paired-device dedup (supersede-by-install-id, revoke-on-logout) plus a QR-pairing first-start race fix with in-session self-heal.
• Mobile chat — agent busy dots moved into the chat header with an idle state.
• Codex — startup update-check disabled at launch.

Install or update with npm install -g devchain-cli.