Add draftwise scaffold for greenfield file scaffolding

[4nkur]

PR2 of the greenfield expansion — turns the chosen stack's plan into actual files on disk, optionally.

init (greenfield + api mode) now also writes
.draftwise/scaffold.json with the structured stack data:

{
  "stack": "Next.js + Postgres + Prisma",
  "summary": "...",
  "directory_structure": "<markdown tree>",
  "initial_files": [{ path, purpose }, ...],
  "setup_commands": [...]
}

The greenfield agent-mode instruction picks up the same file — the host coding agent is now told to save scaffold.json alongside overview.md so draftwise scaffold works in both modes.

draftwise scaffold:

• Reads .draftwise/scaffold.json. Errors with a helpful message if missing or malformed.
• Lists the planned files with purposes, then asks the user to confirm before writing. The confirm prompt warns that scaffolders like create-next-app should run first, since pre-creating files can interfere with a fresh scaffolder run.
• Default-NO on the confirm to avoid surprises.
• Creates each file with placeholder content tailored by extension (// TODO for JS/TS, # TODO for Python, # heading for markdown, etc.). Recursively creates parent dirs.
• Skips files that already exist (no overwrite).
• Prints setup_commands as a reminder; does NOT execute them.

7 new scaffold tests + 1 init test verifying scaffold.json is written. 102 tests total, all passing.

Per docs-sync: README + CLAUDE.md updated to add scaffold to the command list, mention scaffold.json in the .draftwise/ tree diagram, and check off the "optional file scaffolding" roadmap item.