Improve MNE system prompt: data types and uncertainty handling

[neuromechanist]

Summary

• Add eye-tracking to MNE supported modalities list and document specialized submodules (mne.preprocessing.eyetracking, mne.preprocessing.nirs)
• Replace permissive "always attempt to answer" guidance with explicit uncertainty handling: search before claiming support, say "I don't know" when tools return nothing
• Add eyetracking tutorial to documentation sources (verified URL returns 200)

Test plan

• MNE YAML config tests pass (18 passed)
• Eyetracking tutorial URL verified accessible
• System prompt changes align with issue Improve MNE system prompt: supported data types and 'I don't know' behavior #250 acceptance criteria

Closes #250

[neuromechanist]

PR Review

Code review found 2 important issues, both addressed in follow-up commit:

1. Uncertainty handling too restrictive (confidence: 85) - Original replacement broke consistency with base template and other communities. Could cause assistant to refuse answering obvious questions if tool calls return empty. Fixed by softening to additive verification emphasis while preserving helpfulness.

2. description field not updated (confidence: 82) - Line 6 omitted eye-tracking while system prompt included it. Fixed by updating description to include eye-tracking.