Iter 13d: rustdoc polish for ailang-core + new docwriter agent

Adds ailang-docwriter to /agents/ — a recurring role for keeping
crate-, module-, and pub-item-level rustdoc accurate. First mission:
ailang-core. Crate root, every module root, every pub item documented;
intra-doc links throughout; Iter-13a additions (TypeDef.vars, Type::Con.args)
get an explicit backwards-compat note. Two stale broken-link warnings in
ailang-check fixed in passing. cargo doc --no-deps now warning-free across
the workspace; promoted to verification invariant 6 in DESIGN.md.

crates/ailang-check/src/diagnostic.rs

@@ -3,7 +3,7 @@
 //! A [`Diagnostic`] is the machine-readable representation of a problem
 //! reported by the typechecker (or an upstream load step).
 //! [`Severity`] serializes as a lowercase string
-//! (`"error"` / `"warning"`); [`code`] is a stable kebab-case identifier
+//! (`"error"` / `"warning"`); the `code` field is a stable kebab-case identifier
 //! that tooling can consume without parsing the `message` text.
 //!
 //! Convention: each call to [`super::check_module`] reports at most one

crates/ailang-check/src/lib.rs

@@ -9,7 +9,7 @@
 //! Effects are propagated as a set and reconciled against the annotation
 //! on the function type.
 //!
-//! Built-in operations are resolved via [`Builtins`].
+//! Built-in operations are resolved via the internal `Builtins` table.
 //!
 //! ## Internals
 //!