fix: align __version__ with package version 2.4.0a3

__init__.py reported 2.4.0a2 while pyproject (and the built dist) are
2.4.0a3 — a propagation miss. Corrects runtime-reported version. Note:
PyPI 2.4.0a3 is already published carrying the stale a2 string; this
correction rides the next release (no republish of a3 is possible).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: aeoess

src/agent_passport/__init__.py

@@ -28,7 +28,7 @@
 Docs: https://aeoess.com/llms-full.txt
 """
 
-__version__ = "2.4.0a2"
+__version__ = "2.4.0a3"
 
 # Crypto
 from .crypto import generate_key_pair, sign, verify, public_key_from_private