chore: bootstrap repo scaffolding (M0.1)

Adds the §1.2 scaffolding required by IMPLEMENTATION_PLAN.md M0.1:
README, CONTRIBUTING, CODEOWNERS, CHANGELOG, PR + issue templates,
CI workflow, release workflow, LICENSE, commitlint, cliff config,
.editorconfig, .gitignore, .env.example.

Refs: M0.1

CHANGELOG.md

@@ -0,0 +1,25 @@
+# Changelog
+
+All notable changes to this repo. Format: [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
+Generated section is appended on release tag via `git-cliff` (see `.gitea/workflows/release.yaml`).
+
+## [Unreleased]
+
+### Added
+-
+
+### Changed
+-
+
+### Fixed
+-
+
+### Removed
+-
+
+### Security
+-
+
+---
+
+<!-- Released versions appear below this line, newest first. Don't edit by hand once the release workflow has run. -->