Align onboarding with the current PostgreSQL and importer workflows

[aldoborrero]

Summary

• Remove the unused root SQLite seed script; the PostgreSQL development stack uses migrations and the Rust seed command.
• Replace nonexistent importer CLI commands in the quickstart with the implemented admin-screen workflow, including local startup and user provisioning.
• Distinguish full previews, full commits, and incremental commits, including OAuth refresh behavior during previews.
• Preserve the planned CLI contract and mark its unfinished tasks accordingly.

Validation

• Checked the server's --help output and CLI definitions: no import subcommand exists.
• Compared the instructions with the importer screen's labels and mode/scope handlers, routes, and process-compose configuration.
• Confirmed no remaining references to the removed seed script.
• Formatted and checked all three changed Markdown files with the pinned Nix mdformat package; git diff --check passes.

No Rust, dependencies, SQL queries, or migrations changed. No database was modified and no live Harvest end-to-end import was run for this documentation cleanup. The quickstart's acceptance scenarios remain manual validation expectations, not newly certified test results.