handleUpdate accepted any string in the agent_key allowlist field
without running it through isValidSlug. A client could rename an
agent to "weird:key", which would break router cache exact-segment
invalidation (the cache splits on the last colon for invalidation
matching). Add the slug check inline after the allowlist filter and
return MsgInvalidSlug on failure. The slug regex already rejects
colons, slashes, whitespace, and other characters that would confuse
path rendering or cache key parsing — add a dedicated predicate test
covering the full trap surface.
