Added agent developer guide.

Added agent developer guide.

The new top-level AGENTS.md (with CLAUDE.md symlinked to it) is the
canonical index for agent and contributor instructions. Detailed
guides live under docs/agent/:

  docs/agent/engine-dev.md   building, testing, debugging the engine
                             and the nginx modules.
  docs/agent/js-dev.md       writing JavaScript for either engine,
                             with the common nginx API surface and
                             engine differences.
  docs/agent/js-dev-njs.md   specifics of the deprecated njs engine
                             and migration to QuickJS.