feat: align docs surface to the v2 design contract

[Delqhi]

Summary

• closes docs: overhaul OpenSIN docs design toward OpenAI/GitHub/Apple/Uber #147
• align OpenSIN-documentation with the v2 OpenAI + GitHub + Apple + Uber inspired design contract
• add a shared docs/.vitepress/theme/style.css token layer and keep custom.css as the OpenSIN-specific override layer
• normalize homepage and docs custom layouts to shared contract tokens instead of legacy hardcoded colors
• regenerate the tracked VitePress output after the theme updates

Validation

• bun ./scripts/build-docs.mjs
• lsp_diagnostics on docs/.vitepress/theme (no errors)

[Delqhi]

Heads-up: PR #149 complements this design-overhaul branch by isolating nested worktree paths at the Git, CI, and VitePress layers. Once the local nested repos are moved out of the docs repo root, the new predev / prebuild hygiene guard will keep this surface work stable and stop recursive scan / watcher noise from unrelated repos.

[Delqhi]

Launch-readiness note: this branch is a major part of the production docs surface, but current recommendation is still NO-GO until the repository-level security blocker is cleared and the launch-critical PRs are merged to main.

Required before market launch:

• clear the tracked-credentials blocker
• merge chore: document docs repo worktree hygiene #149
• merge this PR
• verify Cloudflare production deploy from main
• run final live smoke on docs.opensin.ai

[Delqhi]

Launch-sequencing note: PR #151 is now open as the repo-side mitigation for blocker #150. Recommended merge order is #151 -> #149 -> #148, then deploy from main and perform a final live smoke on docs.opensin.ai.