docs/emitters: manage the README as blocks the user can edit around #705

Problem

The keep regions of docs/emitters/write-integrate.md protect hand-written islands in generated code. A README is the opposite case: mostly hand-written, with generated sections (installation, authentication, per-feature examples, a table of contents). Overwriting it loses the user's text; not generating it leaves it stale.

Proposal

The docs emitter returns README sections with stable IDs; the engine's integrate step merges them into the existing README between markers, keeps everything outside the markers, respects sections the user froze, and rebuilds the table of contents.