Document OS-specific config directory paths in README

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Task summary

The Configuration section in README.md says config lives in ~/.config/commit-echo, but the code uses platform-specific paths (%APPDATA%/commit-echo on Windows, ~/Library/Application Support/commit-echo on macOS). AGENTS.md documents this correctly; README.md should too.

Why this is a good first issue

Documentation-only fix. Contributors can reference getConfigDir() in src/config/store.ts and tests/get-config-dir.test.mjs.

Suggested files or areas

• README.md
• src/config/store.ts — getConfigDir() implementation
• tests/get-config-dir.test.mjs
• AGENTS.md — already has correct paths

Acceptance criteria

• README documents config paths for Linux, macOS, and Windows
• Paths match getConfigDir() behavior
• Wording is clear for users on each platform

Extra context

A small table or bullet list per OS works well here.