backlog-featlisted

Create a new FEATURE ticket in the backlog. ## Input $ARGUMENTS ## Steps 1. **Check** that `BACKLOG/` exists. If not, tell the user to run `/backlog-init` first and stop. 2. **Calculate the next ID** by scanning files (NEVER read INDEX.md for the ID): - List all files in `BACKLOG/FEATURES/PENDING/` and `BACKLOG/FEATURES/DONE/` - Extract all numbers from `FEAT-XXX.md` filenames - Next ID = max number found + 1 (or 001 if no files exist) - Format: `FEAT-XXX` (zero-padded to 3 digits) 3. **Analyze** the description to determine: - Priorite (Critique / Haute / Moyenne / Basse) - Complexite (XS / S / M / L / XL) - Tags (free-form, relevant to the feature) 4. **Check for external documentation needs**: If the feature involves a third-party library, API, or service (e.g. Stripe, Firebase, AWS SDK, a specific framework), note in the ticket that the relevant documentation should be consulted before starting implementation. Add a line in the description: `📖 Consulter: [doc URL or doc name]`. 5. **Identify affected files**: Based on the description and your knowledge of the codebase, determine which files will need to be created or modified. List actual paths. For new files, indicate `(new)`. For uncertain candidates, add `(?)`. NEVER leave `(A determiner)`. 6. **Write specific acceptance criteria**: Describe the observable behavior the feature must produce. Each criterion should be independently verifiable. Example: "La commande `/foo` cree un fichier dan