Migrate docs to Starlight and ship Phase 1.1 hardening/polish

[prithvi2206]

Summary

This PR migrates the docs site from Fern-managed output to an Astro + Starlight codebase and includes the Phase 1.1 launch hardening + UI polish pass.

What’s included

• Migrates docs, blog, and changelog content into one Starlight-based site.
• Adds Fern compatibility components for current MDX usage.
• Adds llms.txt and llms-full.txt generation.
• Replaces old Fern publish/preview workflows with Starlight checks.
• Implements canonical collection IA:
  • /blog and /changelog are canonical index pages.
  • /blog/all and /changelog/all are 301 compatibility redirects.
• Adds top/bottom canonical back-links on blog/changelog detail pages.
• Introduces a centralized top-nav contract and polished header/tab styling.
• Redesigns blog/changelog index pages from plain lists to structured card-based indexes.
• Tightens visual consistency for Fern card/accordion/admonition components.
• Extends route-parity checks to assert canonical route behavior.
• Adds smoke tests for nav state, redirects, collection wayfinding, and llms endpoints.

Validation

Executed locally:

• npm run check
• npm run test:smoke
• npm run check && npm run test:smoke

All passed.

Notes

• This is a large migration commit that includes generated migration manifests and static assets required by the new site.
• Visual target is directionally aligned with Promptless docs aesthetics, not pixel-perfect parity.