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.

ail-embed/src/lib.rs

@@ -1,7 +1,7 @@
 //! Lean embedding of an AILang M3-frozen staticlib kernel into a
 //! Rust host. The Rust analogue of the audited C host
 //! `crates/ail/tests/embed/tick_roundtrip.c`: same frozen value
-//! layout (design/contracts/frozen-value-layout.md), same
+//! layout (design/contracts/0006-frozen-value-layout.md), same
 //! `own`-mode contract. ZERO `data_server`/finance knowledge.
 
 use std::ffi::c_void;
@@ -32,7 +32,7 @@ impl Ctx {
         Self(unsafe { ailang_ctx_new() })
     }
 
-    /// `State` box, frozen layout (design/contracts/frozen-value-layout.md):
+    /// `State` box, frozen layout (design/contracts/0006-frozen-value-layout.md):
     /// payload `8 + 2*8` = 24 bytes — tag:i64@0, acc:f64@8, n:i64@16.
     /// `ailang_rc_alloc` sets the `p-8` rc header to 1 and zeroes the
     /// payload, so the tag write of 0 is the single-ctor tag.
@@ -88,7 +88,7 @@ impl Kernel {
     }
 
     /// Folds `prices` via the kernel and returns `(Σ px, count)`.
-    /// `own` discipline (design/contracts/frozen-value-layout.md): each `Tick` is freshly
+    /// `own` discipline (design/contracts/0006-frozen-value-layout.md): each `Tick` is freshly
     /// `ailang_rc_alloc`'d and consumed by the kernel; `st` is
     /// consumed and replaced by the return each step; only the final
     /// return is host-`dec`'d.