When BTerminal is launched from a Claude Code terminal, ~8 CLAUDE*
env vars leak into the sidecar child processes. The claude CLI detects
these as nesting indicators and silently hangs. Previously only
CLAUDECODE was removed; now all CLAUDE-prefixed vars are stripped
in both Node.js and Deno sidecar runners.
Replace Agent SDK stub with working implementation that spawns
claude CLI with --output-format stream-json, manages multiple
sessions via Map<sessionId, ChildProcess>, and forwards NDJSON
events to Rust backend. Supports query, stop, and graceful shutdown.