feat(branch): créer la branche depuis la carte (issue #3)

slugify + branch_name_for (issue-<n>-<slug>, reconnu par #1/#2);
client create_branch; POST /api/issues/{o}/{r}/{n}/branch (nom
libre validé, idempotent si existante); la carte passe en En
cours, journalisée source branch, étiquette synchronisée;
formulaire dans la section Code lié du panneau détail.

app/code_links.py

@@ -7,6 +7,7 @@ a token in a branch name (`feature/12-login`, `issue-12`, `12-fix`).
 """
 
 import re
+import unicodedata
 
 _REF_RE = re.compile(r"(?<![\w#])#(\d+)\b")
 _BRANCH_TOKEN_RE = re.compile(r"(?:^|[-_/])(\d+)(?:[-_/]|$)")
@@ -27,6 +28,26 @@ def branch_issue_number(branch_name: str | None) -> int | None:
     return int(m.group(1)) if m else None
 
 
+def slugify_title(title: str, max_len: int = 40) -> str:
+    """ASCII slug for a branch name: 'Créer la branche !' -> 'creer-la-branche'."""
+    normalized = unicodedata.normalize("NFKD", title or "")
+    ascii_text = normalized.encode("ascii", "ignore").decode("ascii")
+    slug = re.sub(r"[^a-z0-9]+", "-", ascii_text.lower()).strip("-")
+    slug = re.sub(r"-{2,}", "-", slug)
+    if len(slug) > max_len:
+        slug = slug[:max_len].rstrip("-")
+    return slug or "issue"
+
+
+def branch_name_for(number: int, title: str) -> str:
+    """The branch a card would create (issue #3): issue-<n>-<slug>.
+
+    The name is recognized back by branch_issue_number, which is
+    what links the future PR to this card (issue #2).
+    """
+    return f"issue-{number}-{slugify_title(title)}"
+
+
 def find_code_links(
     branches: list[dict],
     pulls: list[dict],