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

docs/architecture/channels-wire.md

@@ -21,6 +21,14 @@ multiplexing on the `BiStream` the channels layer gives it.
 
 8 bytes of header, followed by `length` bytes of opaque payload.
 
+The machine-readable spec is the BAST document
+[`chunk-header.bast.json`](chunk-header.bast.json) (embedded in the
+crate as `channels::wire::CHUNK_HEADER_BAST`). BAST is plain JSON —
+any language can consume it; the `alktype` crate compiles it into
+readers/writers/validators, and future codegen derives
+language-specific implementations. The Rust implementation is
+hand-rolled (the hot path); the BAST document is the contract.
+
 | field | offset | width | meaning |
 |-------|--------|-------|---------|
 | `channel_id` | 0 | 4 (BE) | The logical channel this chunk belongs to. Channel 0 is pre-negotiated as `alk/call` (ADR-036). Channels 1..N are opened dynamically via `channel/open` (ADR-037). |