Docs(fix[contributing]): Drop the subproject count, log the move

why: The opening line said twelve subprojects where settings.gradle.kts
declares eleven, and a hardcoded count is what the upkeep gate and the
slop rule both forbid — it is false the first module that moves. The
branch also owed itself a changelog entry: it moves a file callers link
to, and the rule requiring one ships in this same branch.

what:
- Describe the build without counting it, as the published-artifact rule
  beneath it already does
- Add the Documented entry for the CONTRIBUTING.md move
- Present the changelog categories as the allowed set rather than the set
  already in use; Security has no entries yet

Author: tony

.github/CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Contributing
 
-libtmux for Java is twelve Gradle subprojects plus `build-logic`, an included
+libtmux for Java is a multi-project Gradle build plus `build-logic`, an included
 build holding the convention plugins. The gates below are what a change has to
 pass before it is proposed.
 

.github/WRITING.md

@@ -66,8 +66,8 @@ observe.
 Entries land under `## Unreleased`. The maintainer assigns the version when
 cutting a release, so nothing written here predicts one.
 
-Group under the headings already in use — `Added`, `Changed`, `Fixed`,
-`Removed`, `Documented`, `Security` — newest release first, dated `YYYY-MM-DD`.
+Group under `Added`, `Changed`, `Fixed`, `Removed`, `Documented`, or `Security`,
+newest release first, dated `YYYY-MM-DD`.
 
 An entry opens with a bold clause naming what changed, then gives the prose that
 makes it decidable:

CHANGELOG.md

@@ -12,6 +12,14 @@ production.
 
 ## Unreleased
 
+### Documented
+
+- **`CONTRIBUTING.md` has moved to `.github/CONTRIBUTING.md`.** The policy an
+  agent or contributor reads is now split three ways: `AGENTS.md` routes and
+  carries only what applies to every change, `.github/WRITING.md` covers how
+  this project writes, and `.github/CONTRIBUTING.md` covers how it works. A
+  link to the old path needs updating; GitHub finds the new one on its own.
+
 ## 0.0.1-alpha.6 — 2026-08-16
 
 ### Fixed