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/ailang-codegen/src/drop.rs

@@ -85,7 +85,7 @@ impl<'a> Emitter<'a> {
         out.push_str("  %is_null = icmp eq ptr %p, null\n");
         out.push_str("  br i1 %is_null, label %ret, label %live\n");
         out.push_str("live:\n");
-        // FROZEN ABI (embedding boundary) — see design/contracts/embedding-abi.md.
+        // FROZEN ABI (embedding boundary) — see design/contracts/0003-embedding-abi.md.
         out.push_str("  %tag = load i64, ptr %p, align 8\n");
 
         let n_ctors = td.ctors.len();
@@ -111,7 +111,7 @@ impl<'a> Emitter<'a> {
                 if lty != "ptr" {
                     continue;
                 }
-                // FROZEN ABI (embedding boundary) — see design/contracts/embedding-abi.md.
+                // FROZEN ABI (embedding boundary) — see design/contracts/0003-embedding-abi.md.
                 let off = 8 + (j as i64) * 8;
                 let addr_id = local;
                 local += 1;