protect per-user event namespace and notify grantees on share

[fabracht]

Summary

• add a $DB/u/# read-only rule so publishes to the per-user event namespace are blocked unconditionally, not only when --scoped-events is on (closes scoped events: $DB/u/# namespace unprotected when --scoped-events is off #84); the internal service still publishes there, and the cross-user subscribe restriction stays flag-gated
• route share grant/revoke events to the affected grantee under --scoped-events by treating a _shares event's recipient as its grantee, so a client learns of gained/lost access without polling (closes scoped events: grantee not notified when a resource is shared or unshared #85); with scoped events off, _shares events stay admin-only as before
• both changes are agent-mode only (topic protection and sharing are agent-only; cluster sharing parity is tracked in cluster parity for diagram sharing #75)

Test plan

• topic-rules unit test: $DB/u/# publish blocked, subscribe allowed
• topic-protection tests: user-namespace publish blocked with scoped events off; internal service still publishes
• integration test: event_recipients returns the grantee for a _shares event, and empty (not broadcast) for an unresolved grantee
• full agent suite + clippy green