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-codegen/src/lib.rs

@@ -174,7 +174,7 @@ pub enum Target {
     /// published via `@__ail_tls_ctx` around the unchanged internal
     /// call. The ctx-threaded C signature is the M2 shape; the
     /// value/record layout is frozen as of M3
-    /// (design/contracts/frozen-value-layout.md).
+    /// (design/contracts/0006-frozen-value-layout.md).
     StaticLib,
 }
 
@@ -687,7 +687,7 @@ fn llvm_scalar(t: &Type) -> &'static str {
         // M3: any other Type::Con reaching an (export) signature is,
         // by the Task-2 export gate, a single-ctor scalar record;
         // it crosses the C ABI as a bare `ptr`
-        // (design/contracts/frozen-value-layout.md — payload pointer).
+        // (design/contracts/0006-frozen-value-layout.md — payload pointer).
         Type::Con { .. } => "ptr",
         _ => "i64",
     }
@@ -2813,7 +2813,7 @@ impl<'a> Emitter<'a> {
     /// allocates an `Ordering` ctor per call via the
     /// `compare__Int` → `match` path, which surfaces as a
     /// `bench_closure_chain` regression. See
-    /// `design/contracts/prelude-classes.md` for the full surface
+    /// `design/contracts/0017-prelude-classes.md` for the full surface
     /// and the Int-only asymmetry rationale.
     fn try_emit_primitive_instance_body(
         &mut self,