docs: add "How It Works" section to README (EN + ZH)

[Copilot]

Adds a bilingual "How It Works" section to both README.md and README_zh-CN.md, explaining dedust's internal pipeline for users who want to understand what happens under the hood.

Changes

• README.md — New section inserted between "Features" and "Installation" covering the 5-stage pipeline with an ASCII flow diagram:

  • Tokenize → Tokenizer produces a typed token stream from raw DSL text
  • Parse → Parser builds structured rule objects (action + target glob + condition tree with and/not/location modifiers)
  • Validate → Validator rejects broad patterns (delete *, delete **/*) lacking conditions
  • Evaluate → Evaluator recursively walks baseDir, applying ignore/skip semantics and resolving spatial predicates (parent, parents, child, children, sibling) per directory
  • Execute → .execute() sorts targets deepest-first and deletes via Node.js fs APIs, capturing errors non-fatally
  • Includes a callout clarifying the default dry-run behaviour

• README_zh-CN.md — Full Chinese translation of the same section, placed at the same position