docs: Official documentation website (Starlight)

[raiseCatError]

Refs #304, Refs #305

This PR introduces the official baseline documentation website for notMyShell, built with Astro and Starlight.

Summary of Updates & Corrections:

• Roadmap & Version Reconciliation:
  • Reconciled the roadmap to accurately track the v0.18.0 release candidate (PR Release v0.18.0: Context Engine, Managed Agents & Workspaces #345 on release/v0.18.0), which unites the Context Engine platform (NMSh Native module ecosystem and upstream module ports #305), declarative Context Packs, capability scheduling, Claude Code status-line context, Status Strip 2.0, git worktree management (/worktrees), GitHub workspace views (/github), and managed agent full-height workspaces (/claude, /ai).
  • Separated implemented-unreleased features (slated for v0.18.0) from future backlog scope (remote pack registry, sandboxed executable extension tier, Nushell/PowerShell backends, lone ESC[3J handling).
• Dynamic Availability Component:
  • Refactored <Availability /> to accept dynamic version and branch properties instead of hardcoding release strings.
• Configuration & Storage Paths:
  • Documented exact platform paths defined in src/configuration/paths.ts: macOS (~/Library/Application Support/notMyShell/config.json) and Linux (~/.config/nmsh/config.json or $XDG_CONFIG_HOME/nmsh/config.json).
  • Documented all 18 real portable category IDs from src/configuration/portability.ts and NEVER_EXPORTED fields.
• Context Engine Modules:
  • Reconciled the module catalog against the 7 bundled first-party pack definitions in src/context/packs/builtin/ and separated them cleanly from core built-in prompt modules.
• Commands & Shortcuts:
  • Full source-backed CLI flags, maintenance subcommands, and slash commands reference.
• Deployment Workflow:
  • Production deployment in .github/workflows/deploy.yml runs strictly on pushes to master and manual workflow_dispatch.
• Branding & Clean Attribution:
  • Official vector assets only; zero legacy media or placeholder art.
  • Zero model, generator, or tool attribution lines in website pages, READMEs, configs, and git commits.

Verification Evidence:

• Local Site Build: npm run build passed cleanly in 5.26s; Pagefind search indexed 18 HTML pages.
• Whitespace / Diff Check: git diff --check master...HEAD returned 0 errors.
• CI Status: All 10 required PR CI checks on GitHub Actions passed completely green (Verify 22.x/26.x, Quality, Matrix Tests on macos-latest and ubuntu-24.04, Code change detection).
• Scope Isolation: PR diff is strictly isolated to site/ and .github/workflows/deploy.yml.