feat(schema): tx schema sync — add new source columns to the model #433

Part of #424. Size: M · Depends on: #428

What are you trying to do?

Apply the safe part of a schema check: add the new source columns to the model, like Desktop's Refresh does, without a full re-import.

What do we do today?

tx schema check (#428) reports drift. Fixing it means a tx add … -t DataColumn for each column, by hand.

What would tx do?

tx schema sync [model] [--table <name>]... [--save | --stage] [--dry-run]

• Added columns: creates each added column as a DataColumn with its name (after any M rename), sourceColumn and mapped dataType. It runs through MutationRunner, so --save, --stage, the preview and live-session undo all work. A run is one transaction labelled "schema sync".
• Never removes columns. removed findings are reported with their dependants, and the exit code is non-zero.
• typeChanged: only applied with --types, and listed in the preview.
• notSelected (the M projects the column out): not added. It reports the M edit that would be needed. Rewriting M comes later.

Acceptance criteria

• sync followed by check reports no added columns.
• --dry-run matches the mutation preview format of the other modify commands.
• Docs and the command surface snapshot are updated.

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