Instruction file named `skill-instructions.instructions.md` is silently ignored

### Bug

`.github/instructions/skill-instructions.instructions.md` is silently dropped — never appears in the model's system prompt, no warning, no log. Rename to literally anything else (e.g. `skill-table.instructions.md`) with identical content and frontmatter → loads correctly.

Likely collides with the `SKILLS_INSTRUCTIONS` feature flag visible in `~/.copilot/logs/`. If that filename is reserved internally, at minimum emit a warning instead of silently swallowing user files.

### Why I care

I only needed this file in the first place because the CLI truncates the `` list (see #2314, #1130, #2416) — I have 45 skills, only ~31 show up, and the budget is a hardcoded tiny character cap. Meanwhile MCP tool schemas dump 10–100× more tokens into the same system prompt with no cap at all. It's a completely arbitrary penalty on skills.

So the workflow people are forced into — "auto-generate a skill registry file because the CLI won't load my skills properly" — is exactly the workflow that this silent filename filter breaks.

### Fix (any of)

1. Don't reserve user-space filenames; rename the internal feature.
2. Warn visibly when a matching file is found and skipped.
3. Document the reserved names.

### Affected version

1.0.30

Commencez par reproduire le problème avec `.github/instructions/skill-instructions.instructions.md`, puis comparez-le avec le même contenu sous `skill-table.instructions.md` ; examinez `~/.copilot/logs/` à la recherche d’indices de la collision de `SKILLS_INSTRUCTIONS`. C’est terminé lorsque le comportement lié aux noms réservés est supprimé ou qu’un avertissement visible est émis lorsque le fichier est ignoré, et que les fichiers renommé et d’origine se comportent comme documenté.

Rédigé par le modèle d'indexation à partir du texte de l'issue.