Remove logheader callback feature; keep Kanta-colored extra string in default header

docs/database.md

@@ -213,30 +213,11 @@ def resolve_user_key(value: str) -> str | None:
 - By default a transaction is logged with an `action by user` header followed
   by the diff lines.  Added paths are colored green, deleted paths red.
 - `kanta.transaction(..., extra="...")` accepts a display-only string that is
-  appended after the action in the default header (colored by Kanta); it is
-  never persisted in the `ChangeRecord`.
-- Register a `@kanta.logheader` callback to compose a custom header.  Declare
-  any of `action: str`, `user: str`, `extra: str`: Kanta passes the parts
-  with its header colors already applied (missing `user`/`extra` as empty
-  strings), so callbacks only arrange text — no color codes, fallbacks, or
-  padding.  `DictPre`/`DictPost` state dicts and the `Kanta` instance can
-  also be injected.  The callback must be synchronous and return `str | None`.
-- Multiple logheader callbacks are stacked in registration order; the first
-  callback to return a non-`None` result wins.  If all return `None`, Kanta
-  falls back to the default header.  The `user` part has already been through
-  the `logfmt` formatters.
+  appended after the action in the header (colored by Kanta); it is never
+  persisted in the `ChangeRecord`.
 - The header and diff parts can be toggled independently per transaction:
   `kanta.transaction(..., log={"header": True, "diff": False})`.
 
-```python
-@kanta.logheader
-def format_header(action: str, user: str, extra: str) -> str:
-    return f"{user}  {action}  {extra}"
-
-with kanta.transaction(action="update", user="alice", extra="Project X") as data:
-    ...
-```
-
 ## Migrations
 
 - Migration source is configured on `Kanta(...)` via `migrations=`.