#1586 threaded the cycle's resolved source through the synthesize phase so
subagent-written pages land in that source's (source_id, slug) rows. The
patterns phase was not updated and kept the pre-#1586 shape: it stamped a
literal 'default' on every collected ref and compared the reverse-write path
against a literal 'default'.
On a per-source cycle that files the page against the wrong source. The row is
created under 'default' (the child had no source_id to scope its put_page
calls) while the reverse-write drops the file into the named source's checkout,
because source_id === 'default' selects the brainDir/<slug>.md branch and
brainDir IS that source's checkout. Row and file then disagree about which
source owns the page — what doctor reports as multi_source_drift.
Applies the same threading synthesize.ts already uses: PatternsPhaseOpts gains
sourceId, cycle.ts passes cycleSourceId at the patterns call site the way it
already does for synthesize, the child carries SubagentHandlerData.source_id,
and reverseWriteRefs takes the cycle source as its native source. Unset stays
'default', so unscoped callers are unchanged.
