docs: sync AGENTS hierarchy and PR #335 solution after singleton removal

Updates module count in AGENTS.md and src/lib/AGENTS.md from 16/14 to 15
(post-deletion of plugin-singleton.ts). Appends a 2026-05-10 follow-up
section to the PR #335 solution doc noting the singleton was removed and
documenting marker-based idempotency as the current correctness contract.

Also commits the implementation plan with all four units marked complete.
The plan was untracked locally during execution per project convention;
landing it here gives the architectural inversion a visible paper trail.

Author: marcusrbrown

AGENTS.md

@@ -46,7 +46,7 @@ systematic/
 ├── src/
 │   ├── index.ts          # Plugin entry (default export)
 │   ├── cli.ts            # CLI entry (list/convert/config commands)
-│   └── lib/              # 16 core modules (see src/lib/AGENTS.md)
+│   └── lib/              # 15 core modules (see src/lib/AGENTS.md)
 ├── skills/               # 45 bundled skills (SKILL.md format)
 ├── agents/               # 51 bundled agents (6 categories: design/docs/document-review/research/review/workflow)
 ├── docs/                 # Starlight docs workspace (see docs/AGENTS.md)