Simplify augment CLI to minimal preset-driven contract

[xiao312]

Summary

• reduce the public augment CLI to a minimal preset-driven contract
• add preview/apply/json/config round-trip support for augmentation
• add --seed support for reproducible augmentation runs
• add canonical docs/augment.md page and update docs/examples
• add targeted tests for augment plan resolution and JSON/apply behavior

New public augment contract

Required:

• --source
• --mech

Usually required:

• --save for --apply
• --preset
• --target-size

Workflow controls:

• --preview
• --apply
• --json
• --write-config
• --from-config

Optional:

• --seed

Verification

• make verify
• make docs-build