SnapshotStore should respect [dev-assets] for cross-branch reads

[emattia]

Problem

SnapshotStore constructs S3 paths using the current branch directly, bypassing the Asset API's [dev-assets] branch resolution. BuildDatasetFlow on a new feature branch sees zero snapshots even when [dev-assets] branch = "main" is configured.

Partially fixed

IngestFlow now writes to self.prj.write_branch (not read_branch). BuildDatasetFlow reads via self.prj.branch (read_branch). DatasetStore writes to self.prj.write_branch.

Remaining

• Need to verify the fix works end-to-end on a feature branch deployment
• Consider adding a backfill flow for bootstrapping new deployments
• SnapshotStore path construction should align with Asset API branch resolution

See: project-audit/data_accumulation_testing.md