chore: remove dead Copilot inspection code (#1661)

The Copilot-powered Java inspection feature under src/copilot was disabled in #1343 (v0.27.0) by removing its activation entry from extension.ts, but the implementation was left behind as unreferenced dead code. Nothing in the extension imports it, no command/menu contributions reference it, and it never runs.

Remove the entire src/copilot directory. Build and typecheck pass; no dependency or package.json contribution changes are needed (lodash and the telemetry wrapper remain used elsewhere).

Author: chagong