Fixes #540.
Supersedes #541, which was based on the old garrytan/embedding-providers branch and now conflicts with current master.

Plain-English Summary

GBrain now supports multiple named sources. That only works if every page, chunk, tag, and version is written back to the same source it came from.

Before this fix, gbrain sync --source docs could correctly remember that it was syncing docs, but the import/write path could still reconcile content by slug alone. If two sources both had README.md, the named source could accidentally touch default. That is data bleed, and it makes multi-source brains unsafe for real teams.

This PR makes source identity part of the write contract.

flowchart LR
  A["source: docs"] --> B["sync/import"]
  C["source: support-kb"] --> B
  B --> D["page key: source_id + slug"]
  D --> E["pages"]
  D --> F["chunks"]
  D --> G["tags"]
  D --> H["versions"]
  E --> I["search stays scoped to the right source"]

Maintainer Notes

• Threads optional sourceId from full sync and incremental sync into runImport() / importFile().
• Threads optional sourceId through importFromContent() and code-file import.
• Scopes page writes, deletes, chunk writes, tag reconciliation, and page-version creation by (source_id, slug) in both PGLite and Postgres engines.
• Preserves back-compat: callers that omit sourceId still target source_id='default'.
• Keeps empty-string source ids explicit by checking sourceId !== undefined, matching the existing regression expectations.
• Extends version APIs so getVersions() and revertToVersion() can target the same source-aware identity as writes.

Human Walkthrough

If your company has two knowledge folders with the same file names, GBrain should not guess which one you meant. This change makes the folder/source name travel with the document all the way into storage, history, tags, and search chunks.

Review-Fix Pass

Latest review-fix commit: 415ee45.

Fixed Copilot review findings around:

• empty-string sourceId handling;
• source-scoped version history and revert;
• over-broad GetPageOpts docs;
• source-scoped import sync anchors.

Validation

• git diff --check
• bun run verify
• bun test test/sync.test.ts test/import-file.test.ts test/multi-source-integration.test.ts
• focused review-fix rerun: bun test test/sync.test.ts test/pglite-engine.test.ts -> 128 pass, 0 fail
• prior full isolated run: HOME=$(mktemp -d) bun run test -> 3831 pass, 0 fail

Notes

The first full local test run without HOME isolation hit this machine's real ~/.gbrain, causing an unrelated BrainRegistry test to resolve the host brain instead of failing as the test expects. Re-running with a clean HOME matches CI/test isolation and passes.

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