docs: land native dependency and install-hook lifecycle on current main

[ORESoftware]

Summary

Land the reviewed native dependency and install-hook architecture on current documentation main without colliding with documents 24–27 or the reserved universal-interoperability document 28.

Changes

• add docs/29-native-dependencies-and-install-hooks.md;
• add the document-29 index row to README.md;
• record that the manifest/schema contract is now implemented through zed-interfaces#16 / merge e7b9e277dd2729811f293d5405a8fff38c026eaf;
• retain the installer lifecycle as under review in zed-cli#48.

Contract documented

• package- and target-level native requirements and hooks;
• fixed argv package-manager invocation rather than shell interpolation;
• independent consent for native installation, hooks, and builds;
• graph-wide manager intersection and stable de-duplication;
• immutable source CAS plus writable lifecycle staging;
• hook/build/output-validation/cache ordering;
• managed Nix profiles outside derivations; and
• host-mutation prohibition and nativeBuildInputs/buildInputs responsibility inside Nix derivations.

This clean two-file PR supersedes the conflicted document-24 branch in #22. Document 28 remains available for the universal environment interoperability contract.

[linear-code]

DEN-1944