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/ail/tests/show_no_instance_e2e.rs

@@ -2,7 +2,7 @@
 //!
 //! Property protected: calling `print` on a function type fires the
 //! `no-instance` diagnostic with a Show-aware message that
-//! cross-references design/contracts/typeclasses.md, so the
+//! cross-references design/contracts/0013-typeclasses.md, so the
 //! LLM author immediately learns which types ship with built-in Show
 //! and how to declare their own instance for a user type.
 
@@ -40,11 +40,11 @@ fn print_on_fn_type_fires_show_aware_no_instance() {
         no_inst.message
     );
 
-    // Must cross-reference design/contracts/typeclasses.md — that's
+    // Must cross-reference design/contracts/0013-typeclasses.md — that's
     // the canonical contract naming which types ship with Show.
     assert!(
-        no_inst.message.contains("design/contracts/typeclasses.md"),
-        "expected design/contracts/typeclasses.md cross-reference, got message: {:?}",
+        no_inst.message.contains("design/contracts/0013-typeclasses.md"),
+        "expected design/contracts/0013-typeclasses.md cross-reference, got message: {:?}",
         no_inst.message
     );
 }