convention: counter-prefix file naming across docs/specs/, docs/plans/, design/contracts/, design/models/

All 176 files in the four accumulating directories now use a
zero-padded 4-digit counter prefix that reflects creation order
(`NNNN-slug.md`). The counter is assigned per directory in strict
git-log creation order; ties broken alphabetically by original name.
The old `YYYY-MM-DD-` prefix on docs/specs/ and docs/plans/ files is
dropped — the date is recoverable from git log and the counter
carries the ordering.

A file's counter is stable for the life of the file: never reassigned,
never reused, never compacted. Deleted files retire their counter;
subsequent files do not fill the gap. This is the property that lets
cross-references stay literal — refs use the full filename including
the counter (`design/contracts/0007-honesty-rule.md`) so they grep
cleanly and resolve directly without a glob step.

313 cross-references updated across .md/.rs/.toml/.c/.json files
(test pins, include_str! paths, design-INDEX entries, baseline notes,
runtime C comments, inter-contract markdown links incl. bare basename
and `../models/foo.md` forms).

CLAUDE.md gets a new "File-naming convention" section spelling out
the rule and rationale. skills/brainstorm/SKILL.md and
skills/planner/SKILL.md updated so new spec/plan creation produces
counter-prefixed names from the start.

The full test suite (cargo test --workspace) passes.

crates/ailang-surface/src/lib.rs

@@ -4,7 +4,7 @@
 //! [`ailang_core::ast::Module`] values. The JSON-AST in `ailang-core`
 //! remains the canonical, hashable, content-addressed source of truth;
 //! form (A) is the AI-authoring projection optimised for token-efficient
-//! production by LLMs (see `design/contracts/authoring-surface.md`).
+//! production by LLMs (see `design/contracts/0001-authoring-surface.md`).
 //!
 //! ## Modules
 //!

crates/ailang-surface/src/parse.rs

@@ -76,7 +76,7 @@
 //! ```
 //!
 //! Notes on the form (the binding constraints are in
-//! design/contracts/authoring-surface.md):
+//! design/contracts/0001-authoring-surface.md):
 //!
 //! - The `lam` form carries `param-types`, a `ret` type, and an
 //!   optional `effects` clause. The AST stores them per-lambda and

crates/ailang-surface/src/print.rs

@@ -125,7 +125,7 @@ fn write_type_def(out: &mut String, td: &TypeDef, level: usize) {
     }
     // `(drop-iterative)` annotation. Printed last (after
     // every ctor) so the canonical form lands consistent with the
-    // design/contracts/data-model.md example. Omitted when `drop_iterative == false`.
+    // design/contracts/0002-data-model.md example. Omitted when `drop_iterative == false`.
     if td.drop_iterative {
         out.push('\n');
         indent(out, level + 1);