fix: stop associating plain text files

[PathGao]

Fixes #179

Release metadata no longer advertises .txt as a Markpad file association. This prevents installers from replacing the normal Text Document context-menu association while retaining .md and .markdown support.

Depends on #301. Please merge in this order: #286 → #287 → #288 → #293 → #296 → #298 → #300 → #301 → this PR.

Validation:

• Regression test verifies only Markdown extensions are advertised.
• npm run check
• npm test (134 passing)
• cargo test (21 passing)
• Windows installer smoke test remains required for a released package.

[PathGao]

@alecdotdev This is a configuration-only fix for #179 in the single chain. It removes the unintended .txt association and retains only Markdown associations.