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/pull_request_template.md

@@ -0,0 +1,24 @@
+<!-- Thanks for contributing to Ludic! Please fill in the checklist below. -->
+
+## What & why
+
+<!-- What does this change and why? Link the issue: Closes #NN -->
+
+Closes #
+
+## Checklist
+
+- [ ] `bin/x test` passes.
+- [ ] For compiler/runtime changes: `bin/x reseed && bin/x bootstrap-cfree`
+      still reaches the self-hosting fixpoint with no C compiler in the loop.
+- [ ] `ludic-fmt` leaves the touched files unchanged (2-space, LF, UTF-8).
+- [ ] New/changed stdlib symbols are documented under `docs/language/**` and
+      registered in `tools/docgen/inventory.json`
+      (`python3 tools/docgen/check.py` passes).
+- [ ] Commits follow [Conventional Commits](https://www.conventionalcommits.org).
+- [ ] No new C / Python / JS in tooling (Ludic only), and no generated
+      artifacts committed outside `build/` / `bin/`.
+
+## Notes for reviewers
+
+<!-- Anything reviewers should look at closely, or follow-ups deferred. -->