feat(web): idx website and user docs (Astro Starlight)

Landing page plus docs for install, quickstart, config, the stocks and
ownership CLI, output and exit codes, the MCP server and per-agent setup,
agent usage, and an agent-led install page (agents/install.md) that a
coding agent follows from a one-line prompt.

- Every docs page is also served as Markdown at <path>.md; llms.txt and
  llms-full.txt come from starlight-llms-txt; a SKILL.md for skill-based
  harnesses is under /skills/idx/.
- The CLI reference and MCP tools pages are generated from idx --help and
  the tools golden; a new web-generated flake check fails when they drift.
- nix develop .#web provides bun; Cloudflare Pages builds web/ on pushes.
- Package homepage metadata points at https://idx.rasyidanaf.com.
- .cloudflare/ (the cf CLI's account cache) is git-ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/public/favicon.svg

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