docs: sync Development page with source, modernize conventions

docs: sync Development page with source, modernize conventions

- fix retinue_memory_search schema block: limit default 10 -> 50 and
  descriptions synced with src/retinue/provider.py
- correct "cosine-similarity search" to KNN by L2 distance (vec0
  default), matching the retrieval section
- sync project-structure tree with the actual repo (import_holographic.py,
  system_prompt_block.txt, test/, Tools/lib, Open with IntelliJ IDEA,
  Development/index.org itself)
- add file-level drawer (:ID: + :CUSTOM_ID: development) and switch the
  back-link to the exported-anchor form
  (../index.html#outline-container-development)
- AGENTS.org: document the anchor back-link and file-level CUSTOM_ID
  conventions for sub-pages