[codex] Add auto_id tag support

[TheSentinel454]

Summary

Adds support for an auto_id db tag option so a mapped model field can participate in Squalor's generated-id writeback even when table metadata does not mark the column as AUTO_INCREMENT.

Details

• Parses db:\",auto_id\" / db:\"id,auto_id\" into field metadata.
• Treats auto_id fields like auto-increment fields in insert planning for LastInsertId() population.
• Excludes auto_id fields from update and upsert assignment lists, matching existing auto-increment handling.
• Documents the tag and adds regression coverage for tag parsing and insert writeback.

Validation

• go test -vet=off ./...

Note: plain go test ./... still fails on the existing vet/example issue: ExampleTable_Join refers to unknown field or method: Table.Join.

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