feat: add Cursor-style Debug Mode with hypothesis-driven instrumentation

[potb]

Summary by cubic

Adds a Cursor-style Debug Mode for hypothesis-driven runtime debugging with lightweight instrumentation and a local NDJSON log server. Adds frontend CSP guidance to enable localhost logging during browser debugging.

• New Features
  • /debug command with a 7-step workflow and guided template.
  • runtime-debugging skill with server code (port 7777), NDJSON schema, JS/TS/Python/Go instrumentation, region markers for cleanup, and verification steps.
  • CSP detection and handling instructions for frontend debugging.
  • Keyword detection for debug terms (English, Korean, Japanese, Chinese, Vietnamese) that injects a [debug-mode] message.
  • .gitignore updated to exclude .opencode/debug/.
  • 37 tests added/updated across commands, skills, and keyword detector; all passing.

^{Written for commit 165e673. Summary will update on new commits.}