feat(cms): U4 one-command site build + local preview

Add tools/build_site.py: runs the full pipeline in dependency order
(build_content -> build_listings -> seo_inject --all --sitemap ->
build_botlinks -> build_search_index), each as a subprocess so a failing
step aborts the build before anything downstream runs (no partial
deploy). --check verifies non-destructively; --images also runs
optimize_images. This is what CI will run in U5.

Fix a build_content idempotence bug found via U4: the hero and
article-header regions were emitted with leading indentation while the
regex left the template's pre-tag whitespace in place, so indentation
compounded (+16 spaces) on every rebuild. First line is now unindented;
the full build is verified idempotent (identical git tree across two
runs). Added a fixpoint test (re-rendering a page's own output is
byte-identical) so this can't regress.

docs/local-preview.md: build + serve on :8000 in all three languages.
tools/README.md: document build_site + build_content.

26 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/local-preview.md

@@ -0,0 +1,62 @@
+# Local preview
+
+How to build the site from `content/` and preview it on your machine, so you
+can see blog / news / event changes before they go live.
+
+## 1. Build
+
+From the repo root:
+
+```sh
+python tools/build_site.py
+```
+
+This runs the whole pipeline in order (renders detail pages from `content/`,
+rebuilds the News hub + homepage, injects SEO + sitemap, regenerates the
+bot-discovery files, and rebuilds the offline search index). Any failing step
+aborts the build, so a broken step never produces half-built output.
+
+Options:
+
+```sh
+python tools/build_site.py --check    # verify pages match content/, write nothing
+python tools/build_site.py --images    # also convert new images to WebP + fix refs
+```
+
+The build is **idempotent** — running it again with no content change produces
+no file changes.
+
+## 2. Preview
+
+Serve the repo root over HTTP (opening the files directly with `file://` breaks
+absolute paths and the language switcher):
+
+```sh
+python -m http.server 8000
+```
+
+Then open:
+
+- English: <http://localhost:8000/en/>
+- Македонски: <http://localhost:8000/mk/>
+- Slovenščina: <http://localhost:8000/si/>
+
+Blog / news / event pages live under, e.g.,
+`http://localhost:8000/en/blog/<slug>/`.
+
+## 3. Requirements
+
+Python 3 with `beautifulsoup4`, `pyyaml`, and `Pillow` (only needed for
+`--images`):
+
+```sh
+python -m pip install beautifulsoup4 pyyaml pillow
+```
+
+## Notes
+
+- `content/` is the **source of truth** for blog/news/events; the detail pages
+  under `en|mk|si/{blog,news,projects}/` are build output. Edit content in
+  `content/` (or, once it is wired up, the CMS), not the generated pages.
+- See `docs/content-model.md` for the front-matter schema and
+  `docs/plans/2026-08-08-001-feat-no-code-cms-plan.md` for the overall design.