OpenAI's Chat Completions name field requires ^[a-zA-Z0-9_-]{1,64}$.
Real-world sender IDs (Discord alice#1234, numeric Telegram IDs, Slack
U07AB12C3DEF, free-form CLI names) often violate this, so a sanitizer
sits between the raw inbound ID and any wire payload.
Disallowed characters collapse into a single underscore, leading and
trailing underscores are trimmed, and the result is byte-safely
truncated to 64 (output is pure ASCII).
This helper will be wired into the agent pipeline in a later commit.
Refs #2702.
d1e9bb6ae3