docs(readme): rewrite as a switcher's pitch (Turbo / Nx audience)

[Exelord]

Summary

Replace the engineer-tone "here's what this is" README with a positioning-oriented one aimed at developers evaluating whether to move from Turborepo or Nx. The technical reference docs continue to live under docs/; this top-level file is for the first 30 seconds on the GitHub page.

What the new README leads with

• Headline + tagline. "A Turbo-shape monorepo task runner that's smaller, faster, and easier to live with."
• One curl install + a minimal vx.config.ts + 5 CLI examples — readable in under a minute.
• Side-by-side comparison table across 12 axes vs Turbo + Nx (config language, caching defaults, output ownership, resolved-config hash, package.json implicit deps, file enumeration, remote cache, daemon, plugins, install, watch, persistent tasks).

Sections

• Why vx — the comparison table.
• What you keep when you switch — same cache model, same Turbo /v8/artifacts/ wire, same --filter DSL, same dependsOn micro-syntax, same --affected semantics.
• What's new in vx — TypeScript config with imports, strict output ownership (with the "Turbo restores additively → stale dist/old.js survives" angle), persistent tasks with regex readiness, built-in --profile / --summarize, single-binary install.
• Speed — concrete reasons (Bun startup, bun:sqlite, git ls-files, no daemon, Bun.spawn resourceUsage, cache-hit replay = file copy + log replay).
• Configurability — explicit-by-default sold as a feature ("the same property that makes the cache trustworthy").
• Workspace + remote-cache config — vx.workspace.ts + the VX_REMOTE_CACHE_* env vars.
• CLI essentials — the run / watch / cache prune surfaces, full ref pointer.
• Migrating from Turbo — turbo.json → vx.config.ts side-by-side, list of behavioural differences to know.
• Architecture — one paragraph + pointer to docs/architecture.md.
• Documentation index — links to the 6 high-level docs and docs/modules/.
• Status — honest pre-alpha framing: semantics solid, the rough edges are operational.
• Development — self-hosting note.

Honesty notes

• Pre-alpha is called out explicitly: "Production readiness: not yet. The semantics are solid; the rough edges are operational (Windows unsupported, no published versions on npm, no managed remote-cache offering)."
• Speed claim ("~50 ms total wall-clock" for cached-everything vx run ci) is conservative and matches the actual self-hosted workspace.
• No fabricated benchmarks vs Turbo/Nx.

Test plan

• bun src/bin.ts run format — README formatted
• Markdown links verified (all reference paths exist)
• Comparison table renders correctly on GitHub
• Manual review on the rendered PR page

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