Fix broken git submodule: external_tools/gapmind/PaperBLAST

[turbomam]

Problem

All CI tests (3.10–3.13) fail at checkout with:

fatal: No url found for submodule path 'external_tools/gapmind/PaperBLAST' in .gitmodules

This has been broken since at least 2026-03-09. Every PR will show failing tests regardless of what it changes.

Fix

.gitmodules needs a URL entry for external_tools/gapmind/PaperBLAST, or the submodule reference should be removed if it's no longer needed.

Context

Noticed while merging #6 (SHA-pinning claude-code-action for supply chain hardening). The test failures are unrelated to that change.