feat(search): add fuzzy search helpers

[zhyui-1]

Summary / 摘要

Add fuzzy matching support to the search plugin headless helpers and document the plugin API.

Motivation / 背景与动机

• Issue:
• Roadmap (docs/ROADMAP.md): P2 mentions fuzzy search.
• OpenSpec change: N/A - scoped search helper/API documentation update, no breaking change.

This improves the reusable search helper API without changing the CodeMirror search panel behavior.

Changes / 变更内容

• packages/plugin-search:
  • Add SearchOptions.fuzzy for literal ordered-subsequence search.
  • Support fuzzy matching in findSearchMatches.
  • Support fuzzy replacement in replaceAllMatches.
  • Keep regexp behavior unchanged when regexp and fuzzy are both true.
  • Add tests for fuzzy, case-sensitive, whole-word, replace, and regexp precedence.
  • Add package README with usage examples and API notes.
• root scripts:
  • Use corepack pnpm in workspace scripts so build/typecheck commands work consistently in environments where bare pnpm is not on PATH.

Testing / 测试

• corepack pnpm vitest run packages/plugin-search/test/plugin-search.test.ts
• corepack pnpm typecheck
• corepack pnpm test
• corepack pnpm build
• corepack pnpm build:electron-demo

Compliance / 合规自检

• CLA signed
• AI disclosure: implemented with Codex assistance; changes were reviewed and validated locally.
• New dependencies: none.
• No build artifacts committed.
• No secrets / .env / personal vault data committed.

Checklist / 自检清单

• Title follows Conventional Commits.
• Public API changes update package README / types.
• Did not touch live-preview-table.ts.
• New capability / breaking change -> OpenSpec proposal linked.
• Change aligns with project scope.

Screenshots / Recordings

N/A - no visible UI changes.

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