feat: support .config/portless.json

[luojiyin1987]

Summary

Add support for loading project config from .config/portless.json as an alternative to the existing root portless.json.

Closes #268.

What changed

• added .config/portless.json as a fallback config location
• preserved existing precedence:
  1. portless.json
  2. .config/portless.json
  3. package.json#portless
• kept apps path matching relative to the project root, including when config is loaded from .config/
• added tests for precedence, validation, and workspace app matching
• updated README, skills/portless/SKILL.md, and CLI help text

Why

This supports the growing .config/ convention without changing existing behavior for projects that already use portless.json.

Notes

Issue #268 suggests making package.json#portless highest precedence, but the current implementation and tests already define portless.json as higher precedence. This PR keeps the existing behavior and adds .config/portless.json as a non-breaking fallback.

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