docs(client): make the transaction examples self-contained

The transaction snippets referenced a `db` whose defining `with` block
from the Quick Start had already closed by the time a reader reaches
them in the same interpreter, and pasted independently they had no
client at all. Each example now opens its own client context, so it
runs exactly as shown.

Author: polaz

README.md

@@ -66,14 +66,18 @@ with CoordinodeClient("localhost:7080") as db:
 ## Transactions
 
 `db.cypher(...)` commits each statement on its own. To make several statements
-land together, or not at all, run them in a transaction:
+land together, or not at all, run them in a transaction (each snippet opens
+its own client, so it runs as pasted):
 
 ```python
-with db.transaction() as tx:
-    tx.cypher("CREATE (:Person {name: $n})", params={"n": "Alice"})
-    tx.cypher("CREATE (:Person {name: $n})", params={"n": "Bob"})
-    # commits here; an exception anywhere in the block rolls back instead,
-    # leaving neither person in the database
+from coordinode import CoordinodeClient
+
+with CoordinodeClient("localhost:7080") as db:
+    with db.transaction() as tx:
+        tx.cypher("CREATE (:Person {name: $n})", params={"n": "Alice"})
+        tx.cypher("CREATE (:Person {name: $n})", params={"n": "Bob"})
+        # commits here; an exception anywhere in the block rolls back
+        # instead, leaving neither person in the database
 ```
 
 The same surface is on `AsyncCoordinodeClient`, with `async with` and awaited
@@ -82,15 +86,19 @@ statements. When the commit point sits outside a block, drive it by hand:
 ```python
 from contextlib import suppress
 
-tx = db.begin_transaction()
-try:
-    tx.cypher("MERGE (n:Entity {name: $n})", params={"n": "Alice"})
-    applied_index = tx.commit()
-except Exception:
-    # Suppressed so a failing rollback cannot replace the error that caused it.
-    with suppress(Exception):
-        tx.rollback()
-    raise
+from coordinode import CoordinodeClient
+
+with CoordinodeClient("localhost:7080") as db:
+    tx = db.begin_transaction()
+    try:
+        tx.cypher("MERGE (n:Entity {name: $n})", params={"n": "Alice"})
+        applied_index = tx.commit()
+    except Exception:
+        # Suppressed so a failing rollback cannot replace the error that
+        # caused it.
+        with suppress(Exception):
+            tx.rollback()
+        raise
 ```
 
 Requires a CoordiNode server of **v0.5.5 or newer** — the release this client