[POC] Add Data Designer recipe discovery

[oliverholworthy]

Summary

POC layered on top of #612 that lets installed packages register Data Designer recipes without DataDesigner core depending on any specific recipe repository.

External packages can register a named recipe with the data_designer.recipes entry point group. Data Designer can then inspect and execute those recipes through the normal generation verbs:

data-designer recipes list --output json
data-designer recipes show retrieval-sdg --output json
data-designer recipes help retrieval-sdg
data-designer preview --recipe retrieval-sdg -- --input-dir ./docs
data-designer create --recipe retrieval-sdg -- --input-dir ./docs

Design

• Adds the data_designer.recipes entry point group as the external registration mechanism.
• Adds data-designer recipes list, show, and help for human and agent inspection.
• Supports text and JSON output for recipe listing and metadata inspection.
• Lets a recipe expose load_config_builder(params) for execution.
• Lets a recipe module expose build_typer_app() so Data Designer can surface recipe-specific help and structured argument metadata from the same Typer command shape used for execution.
• Keeps argparse parser inspection as a compatibility fallback for simple external recipes, but the POC consumer in DataDesignerPlugins uses Typer.
• Keeps bespoke recipes out of DataDesigner core while making installed recipe packages discoverable.

Review Notes

This PR currently includes the #612 commit because the base branch is in a fork. The recipe-discovery work is the top commit. Once #612 lands, this can be rebased so the diff contains only the recipe registry and inspection layer.

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