feat: seed env files from examples

[pkudinov]

Summary

• Add seedEnvFiles config support for committed .env.example files that create missing local env targets and append missing safe vars without overwriting developer values.
• Add wt env seed [path] with --dry-run and --json, usable from the root worktree or branch worktrees.
• Run the same seed pass from wt new / wt setup before slot-specific env patches, and update README plus the shipped wt skill.

Validation

• pnpm test
• pnpm build
• pnpm lint
• Temp repo CLI smoke test for wt env seed --dry-run --json and wt env seed --json, verifying developer values are preserved and missing vars are appended.