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/proposal.md

@@ -0,0 +1,31 @@
+---
+name: "Proposal"
+about: "Propose new language, stdlib, or runtime surface"
+title: "Proposal: "
+labels:
+  - proposal
+---
+
+## Summary
+
+<!-- One or two sentences: what should exist that doesn't today. -->
+
+## Motivation
+
+<!-- What can't be done cleanly now? Who needs this and why? -->
+
+## Proposed surface
+
+<!-- The namespace/API or syntax you propose. Show it in use. -->
+
+```ludic
+```
+
+## Determinism & backends
+
+<!-- Ludic's runtime is deterministic fixed-point. Does this proposal stay
+     replay-safe? Does it work on native AND web-wasm, or is it host-gated? -->
+
+## Alternatives considered
+
+## Open questions