Route Mother child manifests through SDK contract

[nicabarnimble]

Summary

• Routes Mother runtime child manifest parsing through the SDK-owned patina_sdk::manifest::ChildManifest for canonical identity, ingress, contract, and needs.toys fields.
• Routes patina child install default artifact resolution through patina_sdk::manifest::ChildPackage, removing local target-directory WASM inference.
• Updates in-repo child manifests and test fixtures to satisfy SDK-required child identity fields.
• Hardens the Mother git toy against ambient Git environment variables exposed by hook/test contexts.

Why / Design

• The SDK is now the source of truth for child.toml contract rules; Mother should project SDK facts into its richer runtime model instead of duplicating those rules locally.
• Mother-only extension sections (capabilities, needs.scopes, provides, schemas, metrics, connections) remain parsed from raw TOML because they are runtime policy/config extensions not yet owned by the SDK model.
• Package installs now require the manifest-declared [child.artifact].wasm path unless the operator explicitly passes --wasm; implicit search through target directories is removed.
• Invalid child manifests without SDK-required identity fields now fail closed rather than being normalized by Mother defaults.

Changes

• src/child/internal/mod.rs
  • uses patina_sdk::manifest::ChildManifest for canonical manifest fields
  • keeps raw TOML parsing only for Mother-specific extension fields
• src/main_dispatch/child.rs
  • uses patina_sdk::manifest::ChildPackage for default package artifact resolution
  • adds tests for declared artifact selection and missing artifact declaration failure
• children/*/child.toml
  • adds missing child.version fields to in-repo child manifests
• tests/fixtures
  • updates Mother/child/Slate test manifests to be SDK-valid
• mother/src/toys/git.rs
  • clears ambient Git env (GIT_DIR, GIT_WORK_TREE, GIT_INDEX_FILE, GIT_PREFIX) before running git in a selected repo

Validation

• cargo test child::internal::tests --lib
• cargo test commands::mother::loader::tests --lib
• cargo test registry::tests -p mother
• cargo test child_install_tests
• cargo test --test mother_typed_fixture_conformance
• cargo test --test child_init_typed_default
• cargo test commands::slate::tests::slate_project_preopen --lib
• cargo test commands::mother::daemon::tests::installed_children_use_manifest_name_not_wasm_stem --lib
• cargo test toys::git::tests -p mother
• cargo check -q

Compatibility / Operations

• Existing installed children must have SDK-valid child.name, child.version, and child.kind to load under Mother runtime parsing.
• Package installs without --wasm must declare [child.artifact].wasm in child.toml and the file must exist inside the package.
• Explicit --wasm remains available for operator-provided artifact overrides.

Follow-ups

• Publish/version patina-sdk manifest support when ready so external consumers do not need to rely on source-tree path/git integration.
• Full Slate execution in MCT remains blocked on concrete WIT/WASI/toy host adapters; this PR only completes the old Mother SDK manifest consumption slice.