feat: migrate docs from Starlight to Fumadocs at `/docs`

[hey-granth]

Summary

Current documentation is hosted at codectx.granth.tech (root) on an Astro/Starlight template that is outdated, poorly structured, and does not reflect the project's current state (v0.3.0). This issue tracks a full migration to Fumadocs hosted at codectx.granth.tech/docs, freeing the root for the new landing page.

Goals

• Migrate all existing content from Starlight to Fumadocs
• Restructure content to reflect the actual pipeline: dependency graph, tier ranking, budget-driven inclusion, AST summaries
• Make documentation both human-readable and AI-readable
• Replace template placeholder content with accurate, current information
• Free codectx.granth.tech/ for the new landing page

Proposed Structure

/docs
├── Getting Started
│   ├── Installation
│   └── Quickstart
├── Concepts
│   ├── How codectx Works
│   ├── Dependency Graph and Context Ordering
│   ├── Tier Ranking
│   └── Budget-Driven File Inclusion
├── Reference
│   ├── CLI Reference
│   └── Configuration
└── Changelog

AI-Friendliness Requirements

• Add llms.txt at root (codectx.granth.tech/llms.txt) following the llmstxt.org spec
• Add llms-full.txt: single concatenated plain-text dump of all doc pages, generated at build time
• All content in MDX/Markdown, no JavaScript-gated rendering
• Structured headings on every page, no content buried in tabs

Technical Decisions

• Framework: Fumadocs (Next.js based)
• Search: Orama (client-side, works with static export)
• Deployment: Cloudflare Pages, routed under /docs
• Content source: MDX files under content/docs/

Routing Change

Before	After
codectx.granth.tech	codectx.granth.tech/docs
codectx.granth.tech/installation	codectx.granth.tech/docs/installation
codectx.granth.tech/reference	codectx.granth.tech/docs/reference

Redirects from old URLs to new /docs/* equivalents must be configured
at the Cloudflare Pages level to avoid breaking existing links.

Acceptance Criteria

• All pages from existing Starlight docs migrated with updated content
• /docs loads Fumadocs shell with working sidebar and search
• llms.txt present at codectx.granth.tech/llms.txt
• llms-full.txt generated at build time and accessible
• Concepts section exists and explains the pipeline accurately
• CLI Reference documents all flags and options current as of v0.3.0
• No content references outdated behavior (pre-v0.3.0)
• Redirects configured from all old root URLs to new /docs/* paths
• Passes Lighthouse accessibility score >= 90

Out of Scope

• Landing page (tracked in separate issue)
• Versioned docs
• API reference auto-generation

References

• Fumadocs docs
• llmstxt.org spec
• Current Starlight source: codectx.granth.tech (root)