Add GSP init and AI usage tooling

specs/versions/0.1/ai-usage.md

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+# AI Usage 0.1
+
+- Read `gsp.manifest` first.
+- Treat `design/` as the default GSP source directory.
+- `.gsp` files use YAML.
+- Preserve `id`; do not rename it unless explicitly requested.
+- `id` is the unique identity of a GSP unit.
+- Use only fields valid for the declared GSP version.
+- `with` means related design context.
+- `refines` means single-source refinement.
+- Empty `context` means placeholder.
+- Do not invent missing referenced GSPs silently.
+- Use `gsp validate` after editing GSP files.
+- Use `gsp index` to locate GSP units.
+- Use `gsp trace <id>` to inspect relations.
+- Use `gsp flatten <id>` before implementation or task splitting.
+- Use `gsp pack <id>` when a compact AI context is needed.
+- Use `gsp stage-check --stage <stage>` before stage handoff.