feat(docs): adopt txn2 design system for docs site

[cjimti]

Summary

• Re-skin the docs site to match the txn2 visual identity used by sister project kubefwd. Token-alignment level adoption: keep MkDocs Material chrome, replace the visual layer via extra.css and a custom homepage template.
• New docs/overrides/home.html provides the full-bleed homepage (rail, hero, two flagship cards for the CLI and the Go library, 7-row stack list, coda, custom home-footer). Inner pages still render through Material with the slate scheme remapped to upstream tokens.
• New DESIGN.md records the adoption decisions and defers to upstream txn2/www DESIGN.md and tokens.json as the single source of truth. Includes a brief recap of the MkDocs Material learnings carried over from kubefwd.

What changed

• mkdocs.yml: single slate palette, font: false so CSS loads the upstream Google Fonts URL with trimmed axes (Fraunces ital + opsz/wght, Instrument Sans roman, JetBrains Mono roman).
• docs/overrides/main.html: load the upstream Google Fonts via a single <link>, expand OG and Twitter meta.
• docs/overrides/home.html (new): full homepage template with rail, hero (txeh / /etc/hosts / management.), CLI demo card with real `txeh add` / `txeh list ip` / `txeh remove host` invocations, library demo card with the `NewHostsDefault` / `AddHost` / `Save` path, stack list of features linking to real anchors, coda, home-footer with a `txn2 / org` panel.
• docs/overrides/404.html: rewrite in the design voice.
• docs/stylesheets/extra.css: full port of kubefwd's design system stylesheet (1,536 lines: tokens, blueprint grid, grain/vignette overlays, rail, hero, flagship, terminal, stack, coda, home-footer, plus a complete [data-md-color-scheme=\"slate\"] Material chrome restyle for inner pages, including Mermaid theme variables, code blocks, admonitions, tabbed content, and tables).
• docs/index.md: reduce to a stub with template: home.html.
• DESIGN.md (new): local adoption record. References upstream as canonical.

Test plan

• mkdocs build --strict builds clean
• Homepage renders rail, hero, both flagship cards, terminal blocks, 7 stack rows, coda, home-footer
• Inner pages (e.g. /cli/, /library/) inherit the slate scheme and the txn2 tokens
• No browser console errors on homepage load
• No em-dashes (U+2014) or en-dashes (U+2013) in any new files
• Live UTC clock guarded against navigation.instant re-registration via window.__txehClock sentinel
• Verify GitHub Pages deploy renders correctly after merge

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.51%. Comparing base (872853a) to head (ec2b905).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   88.96%   89.51%   +0.55%     
==========================================
  Files          18       18              
  Lines         761      706      -55     
==========================================
- Hits          677      632      -45     
+ Misses         74       64      -10     
  Partials       10       10              

see 15 files with indirect coverage changes

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