Add Gitea Actions CI + switch to PR-based workflow

- Add .gitea/workflows/ci.yml: typecheck -> lint -> unit -> build -> deterministic
  Playwright e2e against the dev webServer (LLM_STUB=1). No dind/docker/static IP.
- package-lock.json is committed (reproducible `npm ci`); verified npm install is a no-op.
- Add CHANGELOG.md, set version 0.1.0, add .env (gitignored) + .env.example placeholder.
- Document the mandatory feature-branch -> PR -> CI -> auto-merge workflow in
  AGENTS.md + README.md (e2e now runs in CI on every PR).
- Add Playwright html/json reporters (playwright-report/, test-results/e2e.json)
  for the CI E2E-results summary step and the failure artifact upload.

.env.example

@@ -0,0 +1,5 @@
+# Copy to `.env` and fill in real values. `.env` is gitignored; never commit secrets.
+
+# Gitea API token (used for PR / branch-protection automation via the Gitea REST API).
+# Get one from: https://git.kkhost.pl/user/settings/applications
+GITEA_TOKEN=gitea-api-token