API: make delete() return None consistently; artifacts.rename should return the Artifact

[teng-lin]

From the v0.6.0 architecture gap review (2026-05-29).

Problem

Every delete() returns a hardcoded True — it can never return False because failures raise — so the boolean is a meaningless flag:

• src/notebooklm/_notebooks.py:511
• src/notebooklm/_sources.py:624
• src/notebooklm/_artifacts.py:567
• src/notebooklm/_notes.py:201

Separately, rename() returns the updated object for notebooks/sources but returns None for artifacts (src/notebooklm/_artifacts.py:587).

Suggested direction

• delete() -> None everywhere (failures already raise)
• artifacts.rename() -> Artifact (match notebooks/sources)

Pre-1.0 breaking-change window.

[teng-lin]

Split: the rename half of this issue moved to #1255 (rename consistency — return the renamed object + fail loud), which carries the live-API investigation: all rename RPCs return null on success, so the fix is the notebooks.rename re-fetch pattern.

This issue (#1211) now covers only the delete()→None part, which we're leaving alone for now — a return-value change (True→None) can't be cleanly deprecation-warned, and the constant True is harmless.