[coreyt]

Summary

• promote the completed 0.5.15 release notes from Unreleased to a dated section
• surface the opt-in Admin and same-host read-only fleet capability in the README and documentation index
• correct the operations health-response example to the current release version

Validation

• make sync && make verify
• uv run pytest tests/test_documentation_contract.py -q
• uv run mkdocs build --strict
• uv run python scripts/check-version-consistency.py --tag v0.5.15
• uv lock --check
• git diff --check

This PR is documentation-only. It does not create a release tag or publish artifacts.