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.toml

@@ -1,6 +1,6 @@
 [package]
 name = "alktype"
-version = "0.1.0"
+version = "0.2.0"
 edition = "2021"
 rust-version = "1.85"
 license = "MIT OR Apache-2.0"
@@ -9,7 +9,7 @@ repository = "https://git.alk.dev/alkdev/alktype"
 readme = "README.md"
 keywords = ["binary", "jsonschema", "wire-format", "serialization", "layout"]
 categories = ["encoding", "data-structures", "parsing"]
-exclude = [".opencode/", "docs/reviews/", "docs/research/", "docs/sdd_process.md", "Cargo.lock"]
+exclude = [".opencode/", "docs/reviews/", "docs/research/", "docs/sdd_process.md", "Cargo.lock", "AGENTS.md"]
 
 [lib]
 name = "alktype"