fix(core): allow Unicode characters in agent names (#3194)

The agent name validation regex only permitted ASCII letters, numbers,
hyphens, and underscores, silently rejecting agents with non-ASCII names
(e.g., Chinese "项目管理"). Replace the regex with Unicode property
escapes (\p{L}\p{N}) to allow letters and numbers from any script.

Also guard the lowercase naming convention warning so it only fires when
the name contains ASCII letters, since case is meaningless for CJK
scripts.

Fixes #3149