Task: Implement codegen.py CLI scaffold

Create codegen.py as the idiomatic Python entry point for ores.codegen. Implement the complete argparse subcommand structure, package layout, and logging configuration as specified in the survey task. Wire generate and regenerate through to the existing generator.py; stub diff with an informative "not yet available" message. The result is a working CLI that can replace generate_refdata_schema.sh for day-to-day use.

• codegen.py exists at projects/ores.codegen/codegen.py and is executable.
• Package src/codegen/ exists with cli.py, generate.py, list_cmd.py, diff_cmd.py, manifest.py, logging_config.py.
• codegen.py --help shows all four subcommands.
• codegen.py generate --model PATH --profile sql --dry-run prints the output path without writing.
• codegen.py regenerate --component refdata --profile sql --dry-run prints all 10 output paths without writing.
• codegen.py list components lists known components.
• codegen.py list models --component refdata lists all _entity.json files.
• codegen.py diff --component refdata prints a clear "not yet available" message.
• --profile all on an _entity.json model is rejected with an error.
• Venv is bootstrapped automatically on first run.
• Site builds cleanly.

Implemented. Package layout:

projects/ores.codegen/
  codegen.sh                  shell wrapper; bootstraps venv, activates it, cds to repo root
  codegen.py                  pure Python entry point; no venv logic
  src/
    codegen/
      __init__.py
      cli.py                  argparse setup + dispatch
      generate.py             generate and regenerate command implementations
      list_cmd.py             list command implementation
      diff_cmd.py             diff stub (requires unified template)
      manifest.py             component → model directory mappings
      logging_config.py       logging setup

Key behaviours:

• generate and regenerate call through to generator.py functions directly (no subprocess).
• --profile all is rejected for _entity.json (schema) models to prevent the footgun.
• --dry-run resolves and prints output paths without writing.
• diff returns a clear error referencing the unified template task.
• Venv managed by codegen.sh (following compass.sh pattern); codegen.py contains no platform-specific paths.