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.

design/contracts/0007-honesty-rule.md

@@ -0,0 +1,29 @@
+# The honesty rule this ledger holds itself to
+
+`design/` describes what AILang **is now**: schema, semantics,
+invariants, runtime contracts. It is present-tense by construction.
+Two things never belong in a contract or model file:
+
+- **Forward intent** (anything stated as planned, intended, or
+  expected of a future iteration rather than true today) — that
+  lives in the Gitea issue backlog.
+- **History and rationale** (how a prior draft read, what changed
+  since, why one option was chosen, why another was rejected, what
+  was dropped in some iteration) — that lives in `git log` (iter
+  and audit commit bodies). Decision-records are commit-body
+  content, not ledger content; this is the honesty rule it holds
+  itself to.
+
+A cross-reference that does belong stays: it is a formal,
+file-relative Markdown link into the durable tier (`design/` or
+source), enforced by the body-link durability gate in
+[`design_index_pin.rs`](../../crates/ailang-core/tests/design_index_pin.rs).
+A reference that cannot be expressed as such a link is, by that
+fact, the history-or-rationale prose the rule above removes.
+
+The single legitimate exception is a present-tense reserved or
+deliberately-excluded claim that is explicitly and correctly
+labelled. The discriminator is not whether a sentence mentions past
+or future, but whether the document asserts something exists, works, or changed that does not.
+
+Ratified by: `crates/ailang-core/tests/docs_honesty_pin.rs`.