docs: add CONTRIBUTING, code of conduct, and Forgejo templates

Contributor onboarding for the self-hosted toolchain (issue #37):

- `CONTRIBUTING.md`: prerequisites, the bootstrap one-liner, the dev loop
  (`bin/x reseed` -> `bin/x bootstrap-cfree` -> `bin/x test`), stdlib-addition
  guidance, and the code/commit conventions (Conventional Commits, ludic-fmt,
  one-job-per-file, Ludic-not-C/Python/JS for new tooling).
- `.forgejo/issue_template/`: bug, proposal, and cleanup/DX templates.
- `.forgejo/pull_request_template.md`: a checklist covering tests, the
  bootstrap fixpoint, formatting, docs/inventory, and commit style.
- `.forgejo/CODEOWNERS` and a short `CODE_OF_CONDUCT.md`.

Closes #37

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/issue_template/cleanup.md

@@ -0,0 +1,21 @@
+---
+name: "Cleanup / DX"
+about: "Repo hygiene, tooling, docs, or developer-experience improvements"
+title: ""
+labels:
+  - cleanup
+  - dx
+---
+
+## Problem
+
+<!-- What friction or inconsistency exists today? -->
+
+## Proposal
+
+<!-- What to change. Keep it scoped and low-risk. -->
+
+## Acceptance criteria
+
+- [ ]
+- [ ]