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/README.md

@@ -272,8 +272,8 @@ feature flags) or in the downstream alknet crate.
 - `@alkdev/alknet: docs/architecture/` — the source architecture docs
   these were ported from (renumbered from alknet ADR-001..094 to alkcall
   ADR-001..045)
-- `@alkdev/alktype` — the binary struct engine, used for channels chunk
-  header layout and JSON payload schema validation
+- `@alkdev/alktype` — the binary struct engine; compiles BAST documents
+  (e.g. `chunk-header.bast.json`) into readers/writers/validators
 - `@alkdev/pubsub` — the TypeScript EventEnvelope prior art the call
   wire format was derived from