Repair README/docs parity with actual CLI surface

[scottbaggett]

Problem

README and CLI docs are outdated relative to current implementation, creating onboarding and trust friction.

Evidence

Examples:

• README documents dex init / dex config validate, but command help does not expose these commands.
• README still documents npm/node install paths despite Bun-first strategy.
• Option docs drift from current command behavior in multiple sections.

Scope

• README.md
• command help text in CLI where needed.

Acceptance Criteria

• All documented commands/options are runnable in current CLI.
• Bun-first guidance is authoritative and consistent.
• Add command-parity check section or script to prevent future drift.

Test Plan

• Manual smoke for documented examples.
• Link-check + command-parity checklist in PR.

[scottbaggett]

Completed in this hardening pass. Validation: bun test (216 pass, 0 fail), bun run lint, bun run typecheck, and bun audit all pass on 2026-02-22.