Add user_template to wrap user messages with sender context (name, id, username)

Configure via ai.prompt.user_template in config.json using {text}, {full_name},
{user_id}, {first_name}, {last_name}, {username} placeholders. Applied to both
history user messages and the current prompt. No-op when unset.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/config.py

@@ -65,6 +65,7 @@ class AIPromptConfig:
     root: Path
     system_file: str = "system.md"
     parts_dir: str = "parts"
+    user_template: str | None = None
 
 
 @dataclass(slots=True)
@@ -234,10 +235,13 @@ def _parse_ai_config(raw_ai: Any, *, root: Path) -> AIConfig | None:
     prompt_root = Path(str(prompt_raw.get("root", "prompts/ai"))).expanduser()
     if not prompt_root.is_absolute():
         prompt_root = (root / prompt_root).resolve()
+    raw_user_template = prompt_raw.get("user_template")
+    user_template = str(raw_user_template).strip() if raw_user_template else None
     prompt = AIPromptConfig(
         root=prompt_root,
         system_file=str(prompt_raw.get("system_file", "system.md")),
         parts_dir=str(prompt_raw.get("parts_dir", "parts")),
+        user_template=user_template or None,
     )
 
     default_model = str(raw_ai.get("default_model", "")).strip()