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/eq_float_noinstance.rs

@@ -2,7 +2,7 @@
 //!
 //! Property protected: a polymorphic Eq/Ord helper invoked at Float
 //! fires `no-instance` (since Float has neither Eq nor Ord instance
-//! per design/contracts/float-semantics.md) AND the diagnostic message
+//! per design/contracts/0005-float-semantics.md) AND the diagnostic message
 //! cross-references the canonical float-semantics contract so the
 //! LLM author immediately learns the partial-Float story rather
 //! than seeing a bare "no instance" message.
@@ -38,7 +38,7 @@ fn eq_at_float_fires_float_aware_noinstance() {
     // Cross-ref to the float-semantics contract — the LLM author should
     // be pointed at the canonical explanation of partial Float orderability.
     assert!(
-        no_inst.message.contains("design/contracts/float-semantics.md"),
+        no_inst.message.contains("design/contracts/0005-float-semantics.md"),
         "expected NoInstance message to cross-reference the float-semantics contract, got: {:?}",
         no_inst.message
     );