refactor: drop alktype dependency; add BAST doc for chunk header

- Remove alktype from Cargo.toml (its only usage was a thin wrapper
  over jsonschema::options().build())
- Replace alktype::validation::build_validator with direct jsonschema
  in dispatch.rs
- Add docs/architecture/chunk-header.bast.json — the chunk header's
  BAST (Binary Abstract Syntax Tree) machine-readable wire spec
- Embed as channels::wire::CHUNK_HEADER_BAST via include_str! so
  downstream Rust crates can consume it without a file lookup
- Add test asserting the embedded BAST doc is valid JSON and matches
  the wire format
- Update AGENTS.md §10 and implementation-specialist.md: BAST docs
  are the contract; trivial/hot-path formats stay hand-rolled,
  complex formats use the alktype engine or codegen

Verification:
- cargo test: 543 passed, 0 failed
- cargo clippy --all-targets -- -D warnings: clean
- cargo fmt --check: clean
- cargo doc --no-deps: clean
- BAST doc compiles + round-trips against alktype v0.2.0 engine

src/protocol/dispatch.rs

@@ -353,7 +353,7 @@ impl Dispatcher {
                     .registry
                     .registration(&operation_name)
                     .and_then(|r| r.spec.publish_schema.as_ref())
-                    .and_then(|schema| match alktype::validation::build_validator(schema) {
+                    .and_then(|schema| match jsonschema::options().build(schema) {
                         Ok(v) => Some(v),
                         Err(e) => {
                             warn!(