Release v0.2.0: BAST pivot

Bump version to 0.2.0, exclude AGENTS.md from the published crate, and
add a CHANGELOG.md covering the breaking BAST pivot (schema format,
compile signature, validation split) plus bug fixes vs v0.1.0.

Verification:
- cargo test --release: all tests pass
- cargo clippy --all-targets -- -D warnings: clean
- cargo publish --dry-run --allow-dirty: packages as v0.2.0, no collision
- AGENTS.md no longer in cargo package --list; CHANGELOG.md included

Cargo.lock

@@ -27,7 +27,7 @@ dependencies = [
 
 [[package]]
 name = "alktype"
-version = "0.1.0"
+version = "0.2.0"
 dependencies = [
  "jsonschema",
  "serde_json",