ci: add GitHub Actions workflows, remove legacy OpenSpec files

- Add CI workflow: commit-lint, ruff, mypy, pytest matrix, markdownlint
- Add release workflow: go-semantic-release, CHANGELOG promotion, uv build
- Add Dependabot config for action and pip version updates
- Add CHANGELOG.md with initial [Unreleased] entries
- Add markdownlint-cli2 configuration
- Add Home Assistant setup and HACS installation guide
- Remove .github/prompts/, .github/skills/ (unused OpenSpec files)
- Remove openspec/ directory (legacy, no longer relevant)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mikejhill

.github/dependabot.yml

@@ -0,0 +1,15 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    commit-message:
+      prefix: "ci"
+
+  - package-ecosystem: pip
+    directory: /
+    schedule:
+      interval: weekly
+    commit-message:
+      prefix: "build"