refactor(plugin): delete plugin-singleton module after factory revert

The plugInOnce abstraction and its test file are no longer reachable —
src/index.ts stopped importing them in the prior commit, and the consumer
test files (plugin.test.ts and opencode.test.ts) dropped the
_resetPluginSingleton reset calls.

Deletes 227 lines of now-dead infrastructure: the singleton helper, its
five test cases, and the module-level exports. The architectural rationale
lives in docs/brainstorms/2026-05-10-multi-load-plugin-registration-requirements.md
and the implementation plan at docs/plans/2026-05-10-002-refactor-multi-load-plugin-registration-plan.md.

Author: marcusrbrown