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/src/main.rs

@@ -1098,7 +1098,7 @@ fn workspace_error_to_diagnostic(
         // typeclass-coherence diagnostics emitted from
         // `workspace::build_registry`. The codes follow the
         // diagnostic-categories section of
-        // `design/contracts/typeclasses.md`.
+        // `design/contracts/0013-typeclasses.md`.
         W::OrphanInstance {
             class,
             type_repr,
@@ -1568,7 +1568,7 @@ fn walk_term(
 /// Adds the variable bindings introduced by a pattern to `scope` and
 /// returns the names that were freshly inserted (so the caller can roll
 /// them back). MVP-restricted: nested ctor patterns only contain
-/// `Var`/`Wild` — see design/contracts/data-model.md.
+/// `Var`/`Wild` — see design/contracts/0002-data-model.md.
 fn bind_pattern(
     p: &ailang_core::ast::Pattern,
     scope: &mut std::collections::HashSet<String>,