Summary

gbrain dream --phase synthesize --dir <brain-dir> writes the synthesized
reflection/idea pages to the default source, even when <brain-dir> is a
registered source (its .gbrain-source dotfile names the source id). Result:
persistent multi_source_drift warnings in doctor, and the named source's
last_sync never advances ("stale" forever).

Root cause

In src/core/cycle/synthesize.ts, collectChildPutPageSlugs() (~L870)
hardcodes source_id: 'default' for all subagent-written pages. The comment at
L849-854 acknowledges this is a known gap:

Subagent put_page tool schema doesn't expose source_id (subagents are scoped
to a single source); default to 'default' for the current dream-cycle product
behavior. Threading the source_id through reverseWriteRefs guarantees getPage
targets the correct (source, slug) row...

So the brain_put_page write path never honors the resolved source from the
6-tier resolveSourceId chain (--source / GBRAIN_SOURCE / .gbrain-source
/ path-match / brain-default / 'default').

Impact

• doctor → multi_source_drift: pages live at default but their intended
  source is the named brain source.
• doctor → sync_freshness: the named source is flagged stale forever,
  because dream writes land in default and never advance the source's sync.
• Workaround forces operators to git commit && gbrain sync --source <id> to
  re-home the pages, which leaves duplicate copies (one in default, one in the
  named source). Search dedupes by slug so retrieval is unaffected, but
  page_count / chunk_count inflate.

Repro

1. gbrain sources add mybrain --path ~/mybrain (so ~/mybrain/.gbrain-source
   = mybrain)
2. gbrain dream --phase synthesize --dir ~/mybrain
3. gbrain doctor → multi_source_drift lists the new reflection/idea pages as
   intended=mybrain but present at default.

Suggested fix

Thread the resolved sourceId into the subagent put_page write path and through
collectChildPutPageSlugs / reverseWriteRefs, so synthesized pages are
attributed to the correct source at write time instead of the hardcoded
'default'.

Env

gbrain 0.35.8.0, postgres engine, multi-source setup (multiple federated
sources).