[Good first issue] Add a macOS and Linux troubleshooting guide

[Xsoleils]

Problem

The npm installation works across platforms, but the README does not yet explain common macOS and Linux problems such as a missing global npm bin directory, shell PATH setup, Ollama connectivity, or selecting a Chromium executable.

Proposed experience

Add docs/TROUBLESHOOTING.md with a focused macOS and Linux section.

Acceptance criteria:

• Cover npm global install/PATH checks without recommending unsafe permission changes.
• Explain how to verify Ollama and how to set SOLEIL_BROWSER_PATH.
• Include soleil doctor as the first diagnostic step.
• Link the guide from the README and CONTRIBUTING.md.
• Use fake paths and tokens only; never ask users to paste credentials into an issue.

Free-first policy

• This proposal preserves SoleilCode's free-first and approval-by-default principles.