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/bug.md

@@ -0,0 +1,34 @@
+---
+name: "Bug report"
+about: "Something in the compiler, runtime, or tooling behaves incorrectly"
+title: "bug: "
+labels:
+  - bug
+---
+
+## What happened
+
+<!-- A clear description of the incorrect behaviour. -->
+
+## Minimal reproduction
+
+<!-- The smallest .ludic program (or command) that triggers it. -->
+
+```ludic
+```
+
+## Expected vs actual
+
+- **Expected:**
+- **Actual:**
+
+## Environment
+
+- Command used (e.g. `bin/x app foo.ludic --headless`):
+- Target (native macOS / headless / web-wasm):
+- Commit (`git rev-parse --short HEAD`):
+- OS / arch:
+
+## Notes
+
+<!-- Stack traces, generated IR, or anything else that helps. -->