FileAgentTelemetrySink constructor
Creates a sink appending to path (the CLI's is ~/.fah/logs/fa.log).
tag is the correlation id rendered as sid=<tag> on every line —
the CLI's own lines always name their session for shared-log
post-mortems; - (the CLI's no-session marker) when the host has none.
Implementation
FileAgentTelemetrySink(this.path, {this.tag = '-'});