feat: implement init file writer

[rogerchappel]

Summary

• Implement stackforge init <template> <name> as a local file writer for oss-cli, next-app, and python-api scaffolds.
• Preserve --dry-run as a non-writing plan, add --force for explicit overwrites, and support repeated --var KEY=VALUE replacements.
• Add smoke verification covering dry-run, write, overwrite refusal, force overwrite, variable replacement, and scaffold creation.

Verification

• pnpm check
• pnpm smoke:init

Notes

• No network or GitHub calls are performed by the init command.
• Links CrewCmd TSK-68.