Patch <title> in index.html with sitename during build

mdcms build now writes the sitename from config.yml into the <title> tag
of index.html. WhatsApp, Slack, and other link-preview crawlers read the
static HTML without executing JavaScript, so the title must be correct in
the raw file. Previously it was blank (or "MD-CMS" in older templates).

docs/unreleased.md

@@ -4,6 +4,12 @@ Changes merged into `development` that have not yet been released to `main`.
 
 ---
 
+## `mdcms build` patches `<title>` with sitename
+
+`mdcms build` now rewrites the `<title>` tag in `index.html` with the value of `sitename` from `config.yml`. Previously the tag was hardcoded (`MD-CMS`) in older templates, or blank in the starter template, so link previews in WhatsApp, Slack, and other crawlers that read static HTML showed the wrong name.
+
+---
+
 ## Untranslated posts now visible in all categories
 
 **Status:** On `development`, pending release.

mdcms.py

@@ -428,6 +428,19 @@ def validate_assets(site_path: Path, cfg: dict) -> list:
 
 # ─── Core build logic ─────────────────────────────────────────
 
+_TITLE_RE = re.compile(r"<title>[^<]*</title>")
+
+
+def _patch_html_title(site_path: Path, sitename: str) -> None:
+    index = site_path / "index.html"
+    if not index.exists():
+        return
+    html = index.read_text(encoding="utf-8")
+    new_html = _TITLE_RE.sub(f"<title>{sitename}</title>", html, count=1)
+    if new_html != html:
+        index.write_text(new_html, encoding="utf-8")
+
+
 def run_build(site_path: Path):
     """Scan pages/ and posts/, write nav.yml and search.json. Raises ClickException on failure."""
     if not site_path.is_dir():
@@ -507,6 +520,8 @@ def run_build(site_path: Path):
     )
     click.echo(f"  Wrote search.json ({len(live_pages) + len(post_records)} entries)")
 
+    _patch_html_title(site_path, cfg.get("sitename", ""))
+
     pwa_enabled = str(cfg.get("pwa", "no")).lower() in ("yes", "true")
     if pwa_enabled:
         generate_pwa(site_path, cfg)