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/e2e.rs

@@ -2742,7 +2742,7 @@ fn str_clone_drop_balances_rc_stats() {
 /// input bytes; RC stats account for three heap-Str slabs (one
 /// from int_to_str, two from str_clone) and three matching frees.
 /// Pins the "uniform consumer ABI" claim from
-/// design/contracts/str-abi.md — str_clone only reads len + bytes +
+/// design/contracts/0011-str-abi.md — str_clone only reads len + bytes +
 /// NUL, never the rc_header.
 #[test]
 fn str_clone_cross_realisation_uniform_abi() {