Remove dead GitHub skills modal from /explore

[ionfwsrijan]

Problem

The "Fetch Skills" button in the GitHub-skills modal on /explore (explore.html:668) has no JavaScript handler anywhere — script.js only binds #btn-show-github, which does not exist on /explore. The modal markup exists at explore.html:646-672 with no trigger to open it, so it is completely dead UI on a public page.

Fix

Remove the dead GitHub modal block (overlay, username input, error div, Fetch Skills and Cancel buttons) from explore.html. The GitHub skill-import feature remains fully available on the homepage (index.html), which is the only page where it is actually wired up (trigger + OAuth flow + addSkill). The script.js modal-binding and OAuth-callback code already guards against missing elements, so no JS changes are needed.

Files changed

• src/templates/explore.html

Testing

• GET /explore returns 200; rendered HTML no longer contains the github-modal-overlay markup.
• No other templates, tests, or JS reference the removed element ids (github-modal-overlay, github-username, github-modal-error, btn-fetch-github, btn-close-github).
• Full suite: 522 passed, 19 pre-existing unrelated failures, 3 skipped.

Closes #1858

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