feat(site): rebuild the landing page in Astro, deploy via Actions (#2818)

Replace the hand-written single-file page with a real Astro project (site/) and
a GitHub Actions Pages deploy. Minimal, editorial design aligned with Codex /
Claude Code: warm paper bg, near-black type with one restrained accent, Inter
Variable, generous whitespace, a typing terminal demo, and an elevated install
widget in the hero (npm / Homebrew / Download) so the very first thing a visitor
sees is how to install or download — downloads go through the R2 CDN.

Pages source is switched from the legacy branch build to the Actions workflow.

Co-authored-by: reasonix <[email protected]>

.gitignore

@@ -6,6 +6,11 @@
 /reasonix
 /reasonix.exe
 
+# Website (Astro) build output
+/site/dist/
+/site/.astro/
+/site/_preview.png
+
 # Go test/build scratch
 *.test
 *.out