docs(sandbox): document dev-sandbox workflow and wire into AGENTS + README

- docs/dev-sandbox.md — full operator guide: why it exists, paths,
  lifecycle, two activation modes, account setup options, failure modes,
  relationship to install:link, and when to run the integration test.
- AGENTS.md — sandbox-first rule in the change policy, pointing at the
  short five-step flow and the invariant-guarding integration test.
- README.md — add a "Sandbox (for breaking changes)" subsection under
  Installation, between Development and Stable, and flag the symlink
  hot-reload risk in the Development section.

No code changes. Covered by the sandbox test suite shipped in f7effa5.