Add Apple graph commands and query tools

Once now ships an Apple graph model adapted from Bazel rules_apple and Buck2 concepts, covering targets, capabilities, providers, schemas, diagnostics, and typed manifest attributes. Built-in Apple rule metadata lives in a Starlark prelude that lowers into Once’s Rust graph schemas, while once.toml remains the user-authored manifest surface. New CLI commands let you build, run, and test Apple targets through a cache-backed graph that materializes artifacts under .once/out//, and inspect the graph with once query targets, once query capabilities, and once query schema. Documentation is available in docs/guide/apple-graph.md.