#110 Reserved index names can overwrite section indexes (releases/issues/pulls/wiki)

Problem

Generated index filenames are not reserved. A hostile .forge can overwrite the section index with a detail page:

• release version: index writes releases/index.html (the release list) after the index and replaces it with the release page;
• issue id: index writes issues/index.html after the issues index;
• PR id: index writes pulls/index.html after the pulls index;
• wiki file index (no extension) writes wiki/index.html after the wiki index.

All four are accepted by the current is_safe_segment/is_safe_id checks. Verified with fixtures; e.g. a release named index makes releases/index.html render <h1>Index release</h1> instead of the release table.

Impact

Hostile or accidental metadata silently destroys a section index, its navigation target, and the API/HTML consistency around it. No script injection, but a denial/correctness bug in the generated site.

Fix

Reserve generated names (index) for releases, issue/PR ids, and wiki pages, or write indexes last and reject collisions loudly. Add one fixture covering all four collisions.