Add target extension contract

[rickardvh]

Summary

• Add a checked command-generation/target-extension/v1 schema and public API for target extension contracts.
• Validate that targets do not own product operation semantics or require per-operation feature maintenance.
• Add support-declaration matrix projection for implemented targets.
• Document the target-extension boundary in the README.

Validation

• uv run pytest tests/test_public_api.py -q
• uv run ruff check src tests
• uv run pytest -q

Fixes #14

[rickardvh]

This is the right ownership direction for command-generation#14. The target-extension contract belongs in command-generation, and the explicit constraints that targets must not own product semantics or per-operation feature maintenance are exactly the boundary AW needs to consume.

One concern before merge: the schema appears to exist in two places, schemas/target_extension.schema.json and src/command_generation/schemas/target_extension.schema.json. If both are source-owned copies, this creates drift risk for the contract that is supposed to become authoritative.

Please either:

• make one location the clear source of truth and generate/copy the other from it; or
• add a check/test proving the two copies remain byte-for-byte or semantically identical.

The important point is that consumers should not have to guess which target-extension schema is authoritative.