Add community infrastructure: issue + PR templates, CoC

Gitea issue templates (.gitea/issue_template/):
- bug.yml: structured form requiring version, environment,
  what-happened, repro steps, expected, optional logs. Routes
  security reports to security@librete.ch instead of public
  issues.
- feature.yml: prompts for the underlying problem before the
  proposed solution, plus alternatives and out-of-scope.

Pull request template (.gitea/PULL_REQUEST_TEMPLATE.md):
checklist for tests, lint, manual exercise, docs, and changelog.
Asks for explicit reviewer notes so trade-offs surface in the
PR description rather than being lost in chat.

CODE_OF_CONDUCT.md: links to Contributor Covenant 2.1 verbatim
rather than inlining; documents scope, reporting address
(conduct@librete.ch), and points enforcement at the Covenant's
own Enforcement Guidelines.

README links the docs/ tree, CONTRIBUTING, and the CoC so new
contributors find the entry points.

Closes #31.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CODE_OF_CONDUCT.md

@@ -0,0 +1,26 @@
+# Code of Conduct
+
+This project adopts the **Contributor Covenant**, version 2.1,
+verbatim. The full text is published at
+[contributor-covenant.org/version/2/1/code_of_conduct/](https://www.contributor-covenant.org/version/2/1/code_of_conduct/)
+and is the authoritative version that governs participation in
+this project.
+
+## Scope
+
+The Contributor Covenant applies in all project spaces — issues,
+pull requests, code review, the project's communication
+channels, and any official events.
+
+## Reporting
+
+To report unacceptable behaviour, contact the project maintainers
+at `conduct@librete.ch`. All reports will be reviewed and
+investigated promptly and fairly. The maintainers are obliged to
+respect the privacy and security of the reporter.
+
+## Enforcement
+
+Maintainers will follow the Contributor Covenant
+[Enforcement Guidelines](https://www.contributor-covenant.org/version/2/1/code_of_conduct/#enforcement-guidelines)
+when responding to any reported incident.