Implemented the first Pod-layer feature contribution registry slice. Added pod::feature with descriptors, contribution registrars, host-authority model, service/background descriptor skeletons, notification/alert/diagnostic sinks, descriptor-approved contribution checks, once-materialized tool registration, and installed Task tools through the normal Worker tool path. External plugin loading, real approval resolver, full background/service runtime, and WorkItem/MCP integrations remain follow-up scope. Final external review approved. Merge validation passed: cargo test -p pod --lib feature::tests --no-fail-fast, cargo test -p pod --lib, cargo test -p llm-worker --lib, cargo fmt --check, cargo check --workspace --all-targets, ./tickets.sh doctor, git diff --check, nix build .#yoi, ./result/bin/yoi pod --help.
